docs: add CHANGELOG.md and "Stability commitments" page for 1.0

[doubleailes]

Summary

Last two items on the Option A 1.0 checklist:

1. ✅ Variant-index parity enforced — test(resolver): enforce variant-index parity in the rez differential #81.
2. ✅ `intersection_priority` (Solver: implement VariantSelectMode::intersection_priority #63) — feat(resolver): implement VariantSelectMode::intersection_priority (closes #63) #82.
3. This PR: `CHANGELOG.md`.
4. This PR: "What 1.0 promises" doc page.

`CHANGELOG.md`

Follows Keep a Changelog conventions. Covers every rc release back to rc.1, plus a forthcoming 1.0.0 entry capturing the variant-index parity (#81) and `intersection_priority` (#82) work. Includes compare-links pointing at the conventional tag names.

Each entry is specific about which PRs / commits delivered the change so a reader can trace any line back to the concrete diff.

`engineering/stability.md`

New docs page articulating exactly what 1.0 commits us to:

• Versioning — workspace + wheel bumped in lock-step; semver as of 1.0; the rc.x series carried no contract.
• Stable API — explicit list of Python and Rust surface covered by semver, plus an equally explicit list of what isn't (the `solver_micro` benches, internal `rez_solver` submodule layout, `Reduction` / failure-detail wording, internal performance characteristics).
• Supported Pythons — 3.9–3.13 today (one abi3-py39 wheel per platform/arch covers them all), 3.14 when pyo3 catches up.
• Supported rez — `pyrer` doesn't import rez; the `PackageData.from_rez` helper is exercised against rez 3.3.0 (vendored submodule) via the 188-case differential.
• Correctness contract — same `(name, version)` set as rez, same variant index, same status — divergence on any is a release blocker.
• What's modelled vs not — single table mapping each rez feature to "supported" or "not supported, fall back to rez."
• Performance is NOT in the contract — the `solver_micro` baselines catch internal regressions, not absolute numbers.
• Breaking-change policy — RFC issue → addition with deprecation → one MINOR cycle → removal at MAJOR.
• Regression reporting flow.

Links

• README gets a new "Release notes & stability" section near the bottom pointing at both.
• The Introduction page's "Next steps" list now links the stability page alongside Engineering notes.

Verification

• `zola build`: clean, 17 pages, 8 sections, no orphans.
• `zola check` reports two "broken external links" against `CHANGELOG.md` on `main`. Both 404 today because the file isn't on main yet; they resolve as soon as this PR merges. CI's docs job uses `zola-deploy-action` (build-only), not `zola check`, so this does not block the build.

1.0 path complete

After this merges, the Option A checklist for 1.0.0 is:

Item	Status
Variant-index parity enforced	✅ #81
`intersection_priority`	✅ #82
`CHANGELOG.md`	✅ this PR
Stability / "what 1.0 promises" doc	✅ this PR

Tagging 1.0.0 is a single `chore(release)` commit away (version bump in `Cargo.toml` + the docs version pill + the Quick Start dep example — same shape as every previous rc bump).

🤖 Generated with Claude Code

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