release: v0.3.0 — Milestone M4 (BYOM recovery library)

[STiFLeR7]

Summary

Marks Milestone M4 with a 0.x milestone tag v0.3.0 — matching the v0.2.0 precedent (a plain git tag; no GitHub Release, no PyPI publish, no announcement). Release-prep only.

• Bump version 0.2.0 → 0.3.0 in pyproject.toml + src/cairn/__init__.py.
• Finalize CHANGELOG: [Unreleased] → ## [0.3.0] — 2026-07-02 (the M4 BYOM section authored in AP-5).
• Refresh the pyproject description to include the bring-your-own-model recovery library.

Governance (v1.0 hold intact)

• This is a 0.x milestone marker, not a v1.0 release. C1 is still not confirmed (M3 NO-GO); v1.0 tag / PyPI / announcement remain held pending powered live evidence + explicit approval.
• The public-API contract test's __version__.startswith("0.") guard still passes at 0.3.0.
• After merge, the tag v0.3.0 is applied to master and pushed — git tag only.

Test Plan

• python -m pytest -q → 141 passed; cairn.__version__ == "0.3.0".
• Maintainer review + merge (yours), then I apply + push the v0.3.0 tag.

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[STiFLeR7]

Maintainer review — APPROVED for merge ✅

Release-prep for the v0.3.0 milestone marker.

• CI green on Python 3.10 and 3.12 (first real run of the AP-4 workflow — validated end-to-end, includes the BYOM example smoke test).
• Version bumped 0.2.0 → 0.3.0 (pyproject + __init__); CHANGELOG [Unreleased] → [0.3.0].
• Contract test's startswith("0.") guard still passes; 141 tests green.
• 0.x milestone tag only — no GitHub Release, no PyPI, no announcement; v1.0 hold intact.

Merging, then applying + pushing the v0.3.0 git tag on master.