feat(kvc): session migration with reset-on-success + direct-append threshold tuning

KVC v2 beats 4DP at ts=1 same-scale on 7/8 metrics:
  TTFT mean -24%, p50 -54%, p90 -64%; lat mean -0.8%, p50 -12.6%, p90 -0.7%.
  Direct-to-D rate jumped 42.8% -> 91.7%. REFACTOR_PLAN_V1 scenario C achieved.

Two-knob fix:
- reset-on-success blacklist decay: clear (sess, D) reject counter on
  successful direct-to-D path. Eliminates v1 thrashing where session 6880
  was stable on decode-1 for 70 turns then collapsed to 75 D-changes after
  cumulative transient pressure tripped the permanent blacklist.
- bump --kvcache-direct-max-uncached-tokens default 2048 -> 8192 via CLI flag.
  41% of v1 fallbacks were 'real-large-append' (>2048 token append); raising
  the threshold lets these go through the direct-to-D fast path.

Code:
- policies.py: RoutingState.session_d_rejects counter + KvAwarePolicy
  migration_reject_threshold; degenerate fallback picks least-rejected D.
- replay.py: record_admission_reject + reset-on-success in _run_request;
  _fallthrough_reason classifies turn-2+ fall-throughs as session-not-resident
  / real-large-append / etc, replacing misleading 'large-append' suffix
  (TEAM_REPORT §2.7).
- cli.py + benchmark.py: --kvcache-migration-reject-threshold flag wiring.

Docs:
- REFACTOR_PLAN_V1_ZH.md: forward-looking plan after ts=1 validation.
- MIGRATION_V1_FINDINGS_ZH.md: v1 thrashing root-cause analysis.
- V2_RESULTS_ZH.md: v2 results, scenario C achievement, attribution.
- TEAM_REPORT_AGENTIC_PD_HYBRID_ZH.md: comprehensive team report.

Scripts:
- sweep_ts1_kvc_n3_plus_dp.sh: ts=1 baseline (KVC 1P3D N=3 + 4DP CA).
- sweep_ts1_migration_v1.sh / v2.sh: validation runs.
- analyze_ts1_validation.py: 4-way comparison analyzer.

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

src/agentic_pd_hybrid/cli.py

@@ -260,6 +260,16 @@ def main() -> None:
         default=2.0,
         help="Cap on per-request backpressure sleep, regardless of D hint.",
     )
+    replay.add_argument(
+        "--kvcache-migration-reject-threshold",
+        type=int,
+        default=3,
+        help=(
+            "Per-(session, D) admission-reject count after which KvAwarePolicy "
+            "skips that D for the session (forces migration). 0 disables. "
+            "See REFACTOR_PLAN_V1 §6.2 / TEAM_REPORT §2.1."
+        ),
+    )
 
     sample = subparsers.add_parser(
         "sample-sessions",
@@ -501,6 +511,16 @@ def main() -> None:
         default=2.0,
         help="Cap on per-request backpressure sleep, regardless of D hint.",
     )
+    benchmark.add_argument(
+        "--kvcache-migration-reject-threshold",
+        type=int,
+        default=3,
+        help=(
+            "Per-(session, D) admission-reject count after which KvAwarePolicy "
+            "skips that D for the session (forces migration). 0 disables. "
+            "See REFACTOR_PLAN_V1 §6.2 / TEAM_REPORT §2.1."
+        ),
+    )
     benchmark.add_argument(
         "--sample-profile",
         choices=["default", "small-append"],
@@ -586,6 +606,7 @@ def main() -> None:
             pool_poll_include_sessions=not args.pool_poll_no_sessions,
             enable_backpressure=args.enable_backpressure,
             backpressure_max_pause_s=args.backpressure_max_pause_s,
+            kvcache_migration_reject_threshold=args.kvcache_migration_reject_threshold,
         )
         results = asyncio.run(replay_trace(config))
         print(
@@ -732,6 +753,7 @@ def main() -> None:
                 pool_poll_include_sessions=not args.pool_poll_no_sessions,
                 enable_backpressure=args.enable_backpressure,
                 backpressure_max_pause_s=args.backpressure_max_pause_s,
+                kvcache_migration_reject_threshold=args.kvcache_migration_reject_threshold,
                 sample_profile=args.sample_profile,
                 min_initial_input_tokens=args.min_initial_input_tokens,
                 max_initial_input_tokens=args.max_initial_input_tokens,