Release v1.2.0

[josephjohncox]

Summary

Release 1.2.0 — a compatible feature release cutting the perf work merged in #19/#20/#21. Bumps pyproject.toml to 1.2.0, regenerates uv.lock, adds docs/releases/1.2.0.md, points the README release-notes link at it, extends the versioning/post-release docs, and moves the pinned version contracts (test_installed_version_matches_project_metadata, test_release_tag_contract_tracks_the_releasable_major_version) to 1.2.0.

The stable 1.0.0/1.1.0/1.1.1 APIs are unchanged; existing applications require no migration.

What ships in 1.2.0

• Selectable locking — synchronized= on RangeSet / create_range_set / BackendRegistry.create (+ RangeSet.synchronized). Default True keeps full thread-safety; False opts out of internal synchronization under a caller-owned contract.
• Fully-native scalar mutators — RangeSet.release(span) -> int and reserve(span, *, require_covered=False) -> int, returning only the changed length, geometry-consistent with add/discard, guarded to authoritative geometry-only sets (no silent fallback).
• Faster add/discard — validation-free native result construction and cached bound native mutators on the cpp_boundary path; MutationResult/Span/IntervalResult unchanged.
• New surface — BackendAdapter.supports_scalar_delta/release_delta_length/reserve_delta_length; RangeSetProtocol.release/.reserve; native IntervalManager.release_delta_length/reserve_delta_length.
• Behavior-preserving perf — geometry-only snapshot cache; lease projection + O(1) fence-lookup caches.

Scoped cpp_boundary evidence (Apple M5 Max): scalar release/reserve ~4.3M/s synchronized, ~5.0M/s unsynchronized; add/discard ~1.5–1.7M/s; raw-native floor ~7.9M/s (was ~0.9M/s for 1.1.1 add/discard). Not a universal claim — see docs/performance.md.

Pre-release gate (run locally on this branch)

• uv lock --check clean; ruff check + format clean; mypy treemendous clean.
• tests/docs + tests/packaging/test_artifact_policy.py: 73 passed (version/notes/link contracts updated).
• uv build → wheel + sdist; scripts.verify_artifact_contents dist rc=0 (docs/releases/1.2.0.md present in sdist); twine check --strict PASSED for wheel and sdist.
• Full suite on the same tree earlier: 1402 passed, 21 skipped.

Publish

On merge, publish a GitHub Release with tag v1.2.0 targeting the merge commit. The release: published workflow re-audits, builds all platform wheels + sdist, runs isolated wheel/sdist smokes and twine check --strict, runs the exact-batch evidence against the immutable pre-promotion commit, and publishes once via the single OIDC trusted publisher.