feat(#71): IMP-42 u1~u5 silent fail chain diagnostics (assert + invalid-char detector + DIAG log)

Stage 4 binding scope — diagnostic-only, fail-loud, sample-agnostic
(RULE 0 / AI-isolation contract). No production behavior change beyond
fail-loud raises on previously-silent failure classes.

u1 src/phase_z2_pipeline.py:2747-2772 — render_slide precondition assert
   (template_id non-empty str + slot_payload dict), placed after the
   `__empty__` short-circuit at 2740 to preserve empty-zone grid behavior.
u2 src/phase_z2_pipeline.py:2681-2710 — _scan_rendered_html_for_invalid_path_chars
   helper covering src / href / url(...) values for backslash, &, '.
   Invoked on partial render (2778) and slide_base assembly (2798).
u3 src/phase_z2_pipeline.py:2638-2676,2733,5509 — _emit_diag_zones_shape
   shape-only [DIAG] JSON at Step 12 slot_payload emit and Step 13
   render_slide entry. No env gate — silence is the bug.
u4 Front/client/src/pages/Home.tsx:388-392 — unconditional [DIAG raw overrides]
   console.log on handleGenerate boundary, after flushUserOverrides() and
   immediately before runPipeline.
u5 tests/phase_z2/test_phase_z2_diag_smoke_general.py — 32-frame general
   smoke driven by load_frame_contracts() registry (not literal MDX 03/04/05),
   parametrizes u1/u2/u3 across the full frame_contracts.yaml top-level.

Tests (Stage 4 verification PASS):
- u1 8 passed, u2 14 passed, u3 12 passed, u4 5 passed, u5 97 passed.
- Backend full regression tests/phase_z2/ 499 passed in 110.84s.
- Frontend full regression 182 passed in 1.10s.

Out of scope (separate axes):
- Path normalization / as_posix migration.
- Autoescape policy change.
- build_layout_css refactor (Stage 1 category-error rejection).
- Recovery / auto-fix on detected invalid path.
- MDX content / frame-selection / zone-composition change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/phase_z2_pipeline.py

@@ -2632,6 +2632,84 @@ def _remeasure_after_frame_reselect(
     }
 
 
+# ─── IMP-42 u3 (#71) — unconditional Step 12 / Step 13 DIAG log helper ──
+
+
+def _emit_diag_zones_shape(stage_label: str, zones_data: list[dict], **extra_fields) -> None:
+    """IMP-42 u3 (#71) — emit shape-only zone metadata to stdout.
+
+    Used at the Step 12 slot_payload emit site and the Step 13 render_slide
+    entry site to make the silent 3-hop handoff visible in the terminal.
+    Shape-only — never logs raw slot_payload values — so the diag is
+    sample-agnostic (RULE 0) and never leaks user content.
+
+    No env gate: silence is the bug this IMP fights, so the log fires
+    unconditionally on every slide loop.
+    """
+    payload = {
+        "zones_count": len(zones_data),
+        "zones": [
+            {
+                "i": i,
+                "position": z.get("position"),
+                "template_id": z.get("template_id"),
+                "slot_keys": (
+                    sorted(z["slot_payload"].keys())
+                    if isinstance(z.get("slot_payload"), dict) else None
+                ),
+            }
+            for i, z in enumerate(zones_data)
+        ],
+    }
+    payload.update(extra_fields)
+    print(
+        f"[DIAG] phase_z2 {stage_label} "
+        + json.dumps(payload, ensure_ascii=False, sort_keys=True),
+        flush=True,
+    )
+
+
+# ─── IMP-42 u2 (#71) — post-render HTML invalid path char detector ──
+
+_INVALID_PATH_ATTR_RE = re.compile(
+    r"""(src|href)\s*=\s*["']([^"']*)["']|url\(\s*['"]?([^)'"\s]+)['"]?\s*\)""",
+    re.IGNORECASE,
+)
+_INVALID_PATH_CHARS = ("\\", "&", "'")
+
+
+def _scan_rendered_html_for_invalid_path_chars(html: str, context: str) -> None:
+    """IMP-42 u2 (#71) — fail loud when rendered HTML asset references
+    contain invalid path characters in src / href / url(...) values.
+
+    Catches three silent fail vectors at the rendered HTML boundary that
+    surface downstream as 404 / asset-load failures far from upstream cause:
+      - Windows backslash from str(Path) (e.g. ``assets\\img.png``).
+      - Autoescape entity ``&`` (raw ``&`` in raw path source string).
+      - Autoescape entity ``'`` (raw ``'`` in raw path source string).
+
+    Raises ValueError on first hit, citing context, attr type, value snippet.
+    Scope-locked to rendered HTML asset attrs only; never inspects CSS grid
+    metadata or static template strings.
+    """
+    for match in _INVALID_PATH_ATTR_RE.finditer(html):
+        if match.group(3) is not None:
+            attr_label = "url(...)"
+            value = match.group(3)
+        else:
+            attr_label = match.group(1).lower()
+            value = match.group(2)
+        for bad in _INVALID_PATH_CHARS:
+            if bad in value:
+                snippet = value if len(value) <= 120 else value[:117] + "..."
+                raise ValueError(
+                    f"render_slide: {context} — invalid path char {bad!r} in "
+                    f"{attr_label} value (value={snippet}). "
+                    "Likely upstream: Windows backslash from str(Path) or "
+                    "autoescape of '&' / \"'\" in raw path string."
+                )
+
+
 def render_slide(slide_title: str, slide_footer: Optional[str],
                  zones_data: list[dict], layout_preset: str,
                  layout_css: dict, gap_px: int = GRID_GAP,
@@ -2651,22 +2729,60 @@ def render_slide(slide_title: str, slide_footer: Optional[str],
             f"render_slide: invalid embedded_mode={embedded_mode!r}; "
             "expected one of 'auto', 'embedded', 'standalone'"
         )
+    # IMP-42 u3 (#71) — unconditional Step 13 entry DIAG log.
+    _emit_diag_zones_shape(
+        "Step 13 render_slide entry",
+        zones_data,
+        layout_preset=layout_preset,
+        embedded_mode=embedded_mode,
+    )
     env = Environment(
         loader=FileSystemLoader(str(TEMPLATE_DIR)),
         autoescape=select_autoescape(["html"]),
     )
-    for zone in zones_data:
+    for zone_index, zone in enumerate(zones_data):
         # Stage 4 Part 2 (Codex #10 Catch N) — empty zone produced by section
         # assignment override has no partial template; render an empty string so
         # the slide_base zones loop preserves grid identity without TemplateNotFound.
         if zone.get("template_id") == "__empty__":
             zone["partial_html"] = ""
             continue
-        partial = env.get_template(f"families/{zone['template_id']}.html")
-        zone["partial_html"] = partial.render(slot_payload=zone["slot_payload"])
+        # IMP-42 u1 (#71) — fail-loud precondition for Step 13 partial render.
+        # Catches the silent fail vector where Step 12 emits a zone dict missing
+        # `template_id` / `slot_payload`. Error message cites zone_index +
+        # missing key so the diag is actionable (vs Jinja TemplateNotFound /
+        # KeyError surfacing far from the upstream emit site).
+        template_id = zone.get("template_id")
+        if not isinstance(template_id, str) or not template_id:
+            raise TypeError(
+                f"render_slide: zones_data[{zone_index}] precondition failed — "
+                f"`template_id` must be a non-empty str, got {type(template_id).__name__}={template_id!r}"
+            )
+        if "slot_payload" not in zone:
+            raise TypeError(
+                f"render_slide: zones_data[{zone_index}] precondition failed — "
+                f"`slot_payload` key missing (template_id={template_id!r})"
+            )
+        slot_payload = zone["slot_payload"]
+        if not isinstance(slot_payload, dict):
+            raise TypeError(
+                f"render_slide: zones_data[{zone_index}] precondition failed — "
+                f"`slot_payload` must be a dict, got {type(slot_payload).__name__} "
+                f"(template_id={template_id!r})"
+            )
+        partial = env.get_template(f"families/{template_id}.html")
+        rendered_partial = partial.render(slot_payload=slot_payload)
+        # IMP-42 u2 (#71) — fail loud on invalid path chars in rendered HTML
+        # asset refs (src / href / url(...)). Catches Windows backslash and
+        # autoescape entity vectors before they reach the browser as 404.
+        _scan_rendered_html_for_invalid_path_chars(
+            rendered_partial,
+            f"zones_data[{zone_index}] template_id={template_id!r}",
+        )
+        zone["partial_html"] = rendered_partial
 
     base = env.get_template("slide_base.html")
-    return base.render(
+    rendered_base = base.render(
         slide_title=slide_title,
         slide_footer=slide_footer,
         zones=zones_data,
@@ -2676,6 +2792,11 @@ def render_slide(slide_title: str, slide_footer: Optional[str],
         token_css=_read_token_css(),
         embedded_mode=embedded_mode,
     )
+    # IMP-42 u2 (#71) — also scan the assembled slide_base output to cover
+    # asset refs introduced by the slide-base shell itself (title / footer /
+    # popup slots) outside the per-zone partial scope.
+    _scan_rendered_html_for_invalid_path_chars(rendered_base, "slide_base")
+    return rendered_base
 
 
 # ─── Selenium check (single slide + per-zone) ──────────────────
@@ -5384,6 +5505,8 @@ def run_phase_z2_mvp1(
         outputs=["step12_slot_payload.json"],
         note="map_with_contract 결과 — actual slot_payload 값 그대로 (key 만 X).",
     )
+    # IMP-42 u3 (#71) — unconditional Step 12 slot_payload emit DIAG log.
+    _emit_diag_zones_shape("Step 12 slot_payload emit", zones_data)
 
     # 6. Build layout CSS — horizontal-2 = dynamic heights (regression preserve), 그 외 = fr default.
     #    Step D-ext : override_zone_geometries 가 들어오면 layout_css 강제.