feat(IMP-15): 실행-4 — debug.json event surfacing + spec taxonomy row

Issue: #48 (IMP-15 실행-4, axis 4: debug.json + spec doc trace).
Parent: #15. Depends on 실행-1/2/3 (events + classifier outputs).

Surfaces the image/table event streams that 실행-1/2/3 already produced
and consumed, mirroring the existing `zone_geometries_px` top-level
precedent (no new pattern introduced). Adds the matching taxonomy row
to the Phase Z fit-classifier/router spec.

src/phase_z2_pipeline.py (+3):
- write_debug_json now lifts `image_events` and `table_events` to
  top-level of `debug.json` via `(visual_runtime_check or {}).get(<k>, [])`,
  exactly mirroring the immediately preceding `zone_geometries_px`
  surfacing line. Defaults to `[]` when `visual_runtime_check` is None
  — additive, no consumer-visible breakage.

docs/architecture/PHASE-Z-FIT-CLASSIFIER-ROUTER-SPEC.md (+1):
- §3.1 taxonomy adds `image_aspect_mismatch` row. Row text explicitly
  marks the signal as post-render `fail_reasons` from Step 14
  visual_runtime_check (rendered vs declared aspect ratio mismatch),
  NOT a router-routed fit_classifier output, and notes the separate
  `image_events` stream surface. Prevents future readers from wiring
  this taxonomy into §3.2 priority list or §4 router action map.

tests/phase_z2/test_debug_json_event_surfacing.py (new, 2 tests):
- `test_write_debug_json_surfaces_image_and_table_events` invokes
  write_debug_json with synthetic visual_runtime_check containing
  both event lists; reads back the on-disk debug.json and asserts
  both keys are present at top level with the exact payloads.
- `test_write_debug_json_defaults_when_visual_runtime_check_none`
  asserts both new keys default to `[]` when visual_runtime_check
  is None — guards the defensive `(… or {})` pattern.

tests/phase_z2/test_spec_taxonomy_image_aspect_mismatch.py (new, 2 tests):
- `test_spec_has_image_aspect_mismatch_row` opens the spec file and
  asserts exactly one `^\| image_aspect_mismatch \|` row exists
  inside the §3.1 table block (no markdown-parser dependency).
- `test_spec_row_marks_post_render_fail_reasons_semantic` asserts the
  row text carries both "Post-render" and "fail_reasons" tokens —
  enforces the Stage 1 guardrail wording.

Verification (Stage 4 PASS, Claude + Codex independent):
- pytest -q tests/phase_z2/test_debug_json_event_surfacing.py \
              tests/phase_z2/test_spec_taxonomy_image_aspect_mismatch.py
  → 4 passed in 0.07s.
- git diff scope: 4 files, +148 insertions / 0 deletions.

Scope-locked: no edits to classifier (실행-3), event generation
(실행-1/2), Step 21 viewer, §3.2 priority list, §4 router action
mapping, or `table_self_overflow` taxonomy row. Pre-existing
dirty/untracked working-tree files left untouched.

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

docs/architecture/PHASE-Z-FIT-CLASSIFIER-ROUTER-SPEC.md

@@ -93,6 +93,7 @@ action :
 | `moderate_overflow` | content_type ∈ {`text_flow`, `frame_label`} AND `line_equivalent` ∈ (1.5, 4] |
 | `minor_overflow` | content_type ∈ {`text_flow`, `frame_label`} AND `line_equivalent` ≤ 1.5 |
 | `hard_visual_fail` | 위 어디에도 매핑 안 됨 OR retry budget 소진 |
+| `image_aspect_mismatch` | Post-render `fail_reasons` signal — Step 14 visual_runtime_check 가 이미지 frame slot 의 rendered aspect ratio 와 declared aspect ratio 불일치를 감지 (router-routed fit_classifier 출력 아님; 별도 image_events stream 으로 표면화) |
 
 ### 3.2 분류 우선순위 (위에서 아래로)