v2.5.2a1 — alpha: refactor seams under mutation proof + agent-facing API polish

Pre-release (opens the 2.5.2 stabilization-patch ladder; latest final release remains v2.5.1, which pip installs by default — pre-releases require explicit opt-in).

First alpha of the 2.5.2 line ("stabilization / bug-fix patch, no new features" — the ladder runs a1 → beta → final). This cut carries the merged alpha-stage work (#59) plus the pre-release review follow-up (#60). DB schema unchanged; tool-contract changes are deliberate, additive, and documented in the tool descriptions (2.5.x charter).

Highlights

• Refactor seams under mutation proof (#59): admin_handlers per-record decomposition (_ImportTally / _MergeTally), vector._search_vector split (_search_vector_remote / local scans), _calibrate_fused_gate extraction — all pinned equivalent to v2.5.1 by a machine-generated golden behaviour harness (tests/behaviour_252.py, 43 equivalence tests) and a mutation-proof script.
• Source contract normalization at the write seam (Task #282): known legacy source shapes (lowercase type words, Rust serde externally-tagged dicts, bare 'user'/'assistant' strings) are normalized to the canonical {type, id, name} on store; unknown shapes stay verbatim and keep surfacing via check_health(invalid_source_type). The fixer is now mapping-based (preserves id/name) instead of the pre-2.5.2 blanket-User overwrite.
• Agent-facing API polish: store responses expose embedded; recall rows expose match_reason; check_health gains a 3-tier status surface.
• Pre-release review follow-up (#60, bug-139..141 in qa/issue-registry.json): check_invalid_source_type(fix=True) reports an additive locked counter so mapped + unmapped + locked reconciles with count; the deliberate null-source → {} write-seam convergence is documented in the store tool description; the no-persist branch of store carries embedded:false (branch on persisted).

Verification

• Full suite: 523 passed, 5 skipped; lint + issue-registry verification green (CI on the tagged commit).
• Independent read-only code review of the full v2.5.1..master diff: APPROVE-WITH-NITS, no release blockers; all three nits fixed in #60.
• PyPI artifacts (wheel + sdist) published via OIDC Trusted Publishing with provenance.