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>

Front/client/src/pages/Home.tsx

@@ -384,6 +384,14 @@ export default function Home() {
       // clicks Generate would race the PUT against /api/run; the u2
       // fallback could then load a stale persisted document.
       await flushUserOverrides();
+      // IMP-42 u4 — unconditional DIAG console.log on the handleGenerate
+      // entry-to-backend boundary. Surfaces the override payload + uploaded
+      // file name so the user can see exactly what crossed the wire when
+      // the pipeline fails silently. No env gate (silence is the bug).
+      console.log("[DIAG raw overrides]", {
+        file: state.uploadedFile.name,
+        overrides,
+      });
       const result = await runPipeline(state.uploadedFile, overrides);
 
       if (!result.success || !result.final_html_exists) {