docs(#95): IMP-95 u11 status-board markers + idempotence/regex tests (docs+test only)

- Add section 9 to PHASE-Z-PIPELINE-STATUS-BOARD.md carving section 3 item (j)
  into 8 IMP-95 sub-axes (j1-j8). j1-j5 = trace-only, j6-j8 = guarded.
- Marker grammar: <!-- IMP-95:<axis> -->VALUE<!-- /IMP-95 --> (distinct from
  IMP-91 grammar so scripts/update_status_board.py MARKER_RE cannot rewrite
  IMP-95 cells).
- Allowed value enum: {pending, trace-only, guarded, active}.
- tests/scripts/test_update_status_board.py: +1 import, +4 module-level
  constants, +3 test functions verifying marker presence/count (8), value
  domain enum, IMP-91 updater isolation against IMP-95 cells, and IMP-95
  regex rewrite idempotence. IMP-91 tests untouched.
- No production-code touched. Default-OFF flag posture preserved; all cells
  trace-only or guarded.

docs/architecture/PHASE-Z-PIPELINE-STATUS-BOARD.md

@@ -203,6 +203,30 @@ Invocation: `python -m scripts.measure_reuse_savings samples/mdx_batch/02.mdx --
 
 ---
 
+## 9. IMP-95 (V4 evidence → B4 `_select_frame` integration) sub-axis markers
+
+> Sub-axis carve-out of section 3 item (j) for IMP-95. Pair-comment markers
+> `<!-- IMP-95:<axis> -->VALUE<!-- /IMP-95 -->`. Closing tag `<!-- /IMP-95 -->`
+> is intentionally distinct from IMP-91's `<!-- /IMP-91 -->` so the IMP-91
+> updater (`scripts/update_status_board.py`) cannot rewrite IMP-95 cells.
+> Allowed values: `pending` (not implemented), `trace-only` (default-OFF flag
+> `PHASE_Z_B4_V4_EVIDENCE`, additive telemetry only — no render-path change),
+> `guarded` (default-OFF regression harness landed and runs locally), `active`
+> (default-ON — not the current IMP-95 target).
+
+| sub-axis | status |
+|---|---|
+| j1 V4-aware selector under `accepted_content_types ⊇` (u2)        | <!-- IMP-95:j1 -->trace-only<!-- /IMP-95 --> |
+| j2 `plan_placement` v4_candidates kwarg + selection_trace (u3)    | <!-- IMP-95:j2 -->trace-only<!-- /IMP-95 --> |
+| j3 Step 11 `placement_trace` hoist (u4)                           | <!-- IMP-95:j3 -->trace-only<!-- /IMP-95 --> |
+| j4 Gatekeeper `v4_short_circuit` telemetry (u5)                   | <!-- IMP-95:j4 -->trace-only<!-- /IMP-95 --> |
+| j5 `partial_exists` precheck (u6)                                 | <!-- IMP-95:j5 -->trace-only<!-- /IMP-95 --> |
+| j6 Flag-OFF SHA parity regression on mdx 01/02/04/05 (u8)         | <!-- IMP-95:j6 -->guarded<!-- /IMP-95 --> |
+| j7 Flag-ON adapter_needed monotone regression (u9)                | <!-- IMP-95:j7 -->guarded<!-- /IMP-95 --> |
+| j8 Flag-ON `placement_trace` field presence regression (u10)      | <!-- IMP-95:j8 -->guarded<!-- /IMP-95 --> |
+
+---
+
 ## 사용 방법
 
 - 새 작업 들어오면 → 본 board 의 *어느 step* 의 status 를 바꾸는 작업인지 식별