docs: mark v1.52.0 released (ROADMAP + continuity + gate notes)

[jamie8johnson]

Post-release bookkeeping for v1.52.0 (mirrors the v1.51.0 pattern, #2091):

• ROADMAP: v1.52.0 released-section added (overlay truth campaign, teardown-abort fixes, pdf embed, windows-check gate, the gate-found watch: completed HNSW rebuild never drained without a next file event — rebuild_in_flight wedges the freshness API #2133 freshness-wedge fix with live-verified unwedge, A/B-passed recall gate).
• PROJECT_CONTINUITY: RELEASING → RELEASED; gate-protocol lessons recorded (no eval+suite concurrency; build the old tag from source for the A/B — the CPU tarball is the wrong instrument); next idle-loop candidates queued.
• notes.toml: two gate surprises (the idle-tick drain invariant, sentiment −1; the A/B instrument protocol, sentiment +0.5).

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