Add top-gap uplift hard gate for raw candidate search (sprint 257)

docs/sprint257_execution_tracker_2026-02-26.md

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+# Sprint 257 Execution Tracker - 2026-02-26
+
+## Scope
+- `sprint257_plan.md`
+
+## Implemented
+
+Pipeline gating enhancement in `tools/mcp/run_sprint_taskitem_pipeline.sh`:
+- Added:
+  - `WSTONE_NATIVE_RAW_TOP_GAP_REQUIRE_UPLIFT`
+- Raw candidate search now hard-fails with exit code `18` when:
+  - uplift is required but weighted top-gap mode is not configured (`WSTONE_NATIVE_RAW_TOP_GAP_WEIGHTED_SELECT=1` + valid backlog file), or
+  - `best_top_gap_score <= baseline_top_gap_score`
+- Raw search summary now emits:
+  - `top_gap_require_uplift`
+
+## Validation Artifacts
+
+Gate OFF (expected pass):
+- `logs/taskitem_runs/01a_fallback_intake_spec_20260226_162632/00_summary.json`
+
+Gate ON (expected fail):
+- `logs/taskitem_runs/01a_fallback_intake_spec_20260226_162633/02ae_raw_candidate_search.json`
+- command exit code: `18`
+
+Observed result on hard sample:
+- `baseline_top_gap_score=0`
+- `best_top_gap_score=0`
+- with required uplift enabled, run blocked as designed.
+
+## Explicit Completion Signal
+
+- Sprint 257: `DONE` (implemented + enforced no-uplift guard for top-gap policy)