feat(#90): IMP-56 u1-u19 catch-up before final close (post-u20 push fix)

u1: text_overrides axis in user_overrides_io
u2: structure_overrides axis in user_overrides_io
u3: vite allowlist for new endpoints
u4: text_override_resolver
u5: Step 12 text_overrides apply in phase_z2_pipeline
u6: structure_override_resolver
u7: text_path_stamper
u8: SlideCanvas text-edit capture
u9: SlideCanvas structure-edit overlay
u10: userOverridesApi service extension
u11: designAgent types extension
u12: slidePlanUtils restore
u13: user_overrides endpoint tests
u14: user_overrides restore tests
u15: pipeline fallback tests
u16: edit-mode state + gating tests
u17: slide_base print mode CSS
u18: /api/connect endpoint (vite)
u19: /api/export endpoint (vite)

Recovery scope: 29 files (12 modified + 17 new). u20 already pushed in
9439575; this commit lands u1-u19 that were authored but not committed
before #90 was externally closed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/structure_override_resolver.py

@@ -0,0 +1,189 @@
+"""IMP-56 (#90) u6 — structure_override resolver (validator + apply).
+
+Step-22 user structure-edit persist axis. Consumed by Step 12 (u7 wiring)
+so a prior render's reorder / hide choices re-apply to the next render
+without re-clicking.
+
+Schema (defined verbatim in ``src/user_overrides_io.py:30`` u2) ::
+
+    structure_overrides = {
+        <zone_id>: {
+            "slot_order":   [<slot_key>, ...],   # optional, partial reorder
+            "hidden_slots": [<slot_key>, ...],   # optional, hide these slot_keys
+        },
+        ...
+    }
+
+SCOPE LOCK (Stage 2 u6 contract, IMP-56 #90 u2 docstring) :
+
+    The only allowed inner keys are ``slot_order`` and ``hidden_slots``.
+    Any other key (e.g., ``frame_id``, ``template_id``, ``unit_id``,
+    ``slot_payload``) is treated as a frame-swap / DOM-rebuild attempt and
+    is DROPPED at validate time. Frame swap stays on the existing
+    ``frames`` axis so the Phase Z no-AI-HTML-structure invariant remains
+    intact. There is intentionally NO escape hatch through this axis.
+
+API (deterministic, no AI) :
+
+- ``validate_structure_overrides(overrides)`` → sanitized copy. Per-entry
+  tolerant (drops malformed rows; never rejects the whole batch — mirrors
+  ``src.text_override_resolver.validate_text_overrides`` u4 contract).
+- ``apply_structure_override(zone, override)`` → ``True`` if the slot-payload
+  mapping was mutated (any hide or any reorder), ``False`` otherwise. The
+  ``zone`` argument is the slot-payload mapping at Step 12 (a mutable
+  mapping whose keys are slot_keys and whose values are typically
+  ``list[str]`` of lines). Identity-preserving: mutates in-place via
+  ``clear`` + ``update`` so caller references remain valid.
+
+Guardrails (refs : Stage 1 binding contract, Stage 2 unit u6) :
+
+- raw_content preservation is a wiring-layer (u7) responsibility — the
+  resolver only ever reorders / removes top-level slot_payload entries.
+  Per-slot ``list[str]`` line content is never inspected or mutated here.
+- AI-isolation : pure deterministic Python; no LLM calls.
+- Carve-out (IMP-46 #62) : brand-new module, does not touch the #76
+  commit ``1186ad8`` cache region.
+"""
+from __future__ import annotations
+
+from typing import Any, Mapping, MutableMapping
+
+
+class InvalidStructureOverride(ValueError):
+    """Reserved for future strict-mode parse errors.
+
+    Currently unused — the resolver follows the u4 per-entry-tolerant
+    contract and silently drops malformed rows at validate time rather
+    than raising. Kept as a public surface so u7 wiring (and future
+    strict-mode callers) can distinguish source-malformation from
+    stale-DOM misses without an API rev.
+    """
+
+
+_ALLOWED_INNER_KEYS: frozenset[str] = frozenset({"slot_order", "hidden_slots"})
+
+
+def _sanitize_slot_list(raw: Any) -> list[str]:
+    """Return a fresh list of non-empty string slot_keys (drop the rest)."""
+    if not isinstance(raw, list):
+        return []
+    out: list[str] = []
+    seen: set[str] = set()
+    for slot in raw:
+        if not isinstance(slot, str) or not slot:
+            continue
+        if slot in seen:
+            # De-dup defensively — a duplicate slot_key in slot_order would
+            # be meaningless (dicts can hold each key once); duplicate in
+            # hidden_slots is redundant. Drop subsequent occurrences.
+            continue
+        seen.add(slot)
+        out.append(slot)
+    return out
+
+
+def validate_structure_overrides(
+    overrides: Any,
+) -> dict[str, dict[str, list[str]]]:
+    """Return a sanitized copy of ``overrides`` (per-entry tolerant).
+
+    Drops:
+      - non-string or empty zone_ids,
+      - non-mapping per-zone payloads,
+      - per-zone inner keys other than ``slot_order`` / ``hidden_slots``
+        (frame-swap attempts are dropped at this gate — see SCOPE LOCK),
+      - non-list ``slot_order`` / ``hidden_slots`` values,
+      - non-string or empty slot_key entries within those lists,
+      - per-zone payloads that contain neither a non-empty ``slot_order``
+        nor a non-empty ``hidden_slots`` after sanitization (empty intent
+        carries no signal).
+
+    Returns a fresh ``dict`` AND fresh nested dicts / lists so callers can
+    use the result as a working buffer without aliasing the persisted
+    payload from ``user_overrides_io.load``.
+    """
+    if not isinstance(overrides, Mapping):
+        return {}
+    out: dict[str, dict[str, list[str]]] = {}
+    for zone_id, mapping in overrides.items():
+        if not isinstance(zone_id, str) or not zone_id:
+            continue
+        if not isinstance(mapping, Mapping):
+            continue
+        zone_out: dict[str, list[str]] = {}
+        for inner_key, inner_value in mapping.items():
+            if inner_key not in _ALLOWED_INNER_KEYS:
+                # Frame-swap attempt or unknown key — drop silently per
+                # SCOPE LOCK. No mechanism through this axis.
+                continue
+            sanitized = _sanitize_slot_list(inner_value)
+            if sanitized:
+                zone_out[inner_key] = sanitized
+        if zone_out:
+            out[zone_id] = zone_out
+    return out
+
+
+def apply_structure_override(
+    zone: MutableMapping[str, Any],
+    override: Mapping[str, Any],
+) -> bool:
+    """Apply ONE structure override to ``zone`` in-place.
+
+    ``zone`` is the slot-payload mapping at Step 12 — i.e. a mutable
+    mapping whose keys are slot_keys and whose values are the per-slot
+    line lists (or other content payload). Mutation is restricted to
+    top-level key membership + ordering; per-slot values are NEVER
+    inspected or modified here.
+
+    ``override`` is the per-zone payload after :func:`validate_structure_overrides`
+    sanitization — i.e. a mapping with only ``slot_order`` and / or
+    ``hidden_slots`` keys, each holding a list of non-empty str slot_keys.
+    This function is also defensive: if non-list values leak through, they
+    are treated as empty (no raise).
+
+    Semantics :
+      1. ``hidden_slots`` are popped first. Entries absent from ``zone``
+         are silently skipped (stale slot_keys from a prior frame).
+      2. ``slot_order`` partially reorders the surviving slot_keys:
+         listed keys (that are present in ``zone``) move to the front in
+         the given order; remaining keys keep their original relative
+         order at the tail. Unknown slot_keys are silently skipped.
+
+    Returns ``True`` if the zone's slot-payload mapping was mutated (any
+    hide that removed a key OR any reorder that changed key order),
+    ``False`` otherwise. Identity-preserving: rebuilds via
+    ``clear`` + ``update`` so the caller's reference to ``zone`` remains
+    valid.
+    """
+    mutated = False
+
+    raw_hidden = override.get("hidden_slots") if isinstance(override, Mapping) else None
+    hidden = _sanitize_slot_list(raw_hidden)
+    for slot in hidden:
+        if slot in zone:
+            del zone[slot]
+            mutated = True
+
+    raw_order = override.get("slot_order") if isinstance(override, Mapping) else None
+    desired_order_seed = _sanitize_slot_list(raw_order)
+
+    current_order = list(zone.keys())
+    desired_order: list[str] = []
+    seen: set[str] = set()
+    for slot in desired_order_seed:
+        if slot in zone and slot not in seen:
+            desired_order.append(slot)
+            seen.add(slot)
+    for slot in current_order:
+        if slot not in seen:
+            desired_order.append(slot)
+            seen.add(slot)
+
+    if desired_order != current_order:
+        snapshot = {k: zone[k] for k in desired_order}
+        zone.clear()
+        zone.update(snapshot)
+        mutated = True
+
+    return mutated