M3 — Consolidate Scout duplicates

Sub-issue of #37124. Scout half of the Rock — mirrors Jose's M3 (#37128). Depends on M0.

Description

Retire the duplication confirmed in M0 across docs/testing/, the docs/core/ slice and the two rules directories — folding each retired file's unique content into the survivor first, not a straight delete. Same method Jose used on the _FOCUSED pair.

Acceptance Criteria

• Every duplication candidate from M0 either consolidated or explicitly ruled out with a reason
• .cursor/rules/*.mdc reviewed for content that restates a doc instead of pointing at it. Both files read so far (test-context.mdc, java-context.mdc) end in an ## On-demand list of @docs/... pointers, which is the right shape — but each also inlines rules that live in those same docs, and java-context.mdc has already drifted (see #37577). Decide per rule: keep inline because Cursor needs it without a file load, or replace with the pointer.
• docs/testing/API_TESTING.md and PERFORMANCE_TESTS.md ruled out as consolidation candidates — both read in full, no overlap with REST_API_PATTERNS.md or INTEGRATION_TESTS.md (see the correction note in #37578)
• Unique content folded into the surviving file before any deletion, verified line by line
• No link left pointing at a deleted file (reachability check from M2 stays green)
• Monitoring gate opened for each merged change

Priority

Medium