Release v0.6.5 — validation & hardening

[ddsha441981]

Cuts the v0.6.5 release. The hardening work itself (fuzz #13, soak #14, Miri #16, shadow-map #18, loom #19, embedded matrix #20, QEMU #21, footprint #22, AccessBuffer drain #24) already landed on main; this is the release-prep commit on top.

Changes

• Cargo.toml — move keywords/categories/readme from [package.metadata.docs.rs] into [package]. They were misplaced, so the crate published to crates.io with zero keywords/categories. Dropped the inaccurate lock-free keyword.
• CHANGELOG — finalize the [0.6.5] heading (2026-09-28) and note the AccessBuffer drain as the one behavioural change, instead of claiming "no logic changes in src/".
• Docs honesty — soften the overstated "lock-free reads" claims (README Known Limitations, concurrency.md, benchmarks.md, core-concepts.md) to the accurate spinlock-free-metadata wording: slab-mode reads (every u64 key) still take the slab pool lock. Left the genuinely-accurate lock-free mentions (AccessBuffer ring buffer, MetaWord atomic loads) and the historical v0.6.2 changelog entry untouched.

Pre-flight (all green on this branch)

• cargo test --all-features — 58 + 11 doc tests, 0 failed
• cargo build --no-default-features (no_std) — clean
• cargo +nightly miri test -p pulse_map --lib — 55 passed, 0 failed, 3 ignored (oversized)
• cargo publish --dry-run — 70 files packaged, verify build OK

Tag v0.6.5 and cargo publish follow after this merges to main.