docs(release): 📝 prepare v0.9.0 changelog and docs sweep (#166)

## Summary

Prepare changelog and documentation for v0.9.0 release. This version
adds opt-in CAS retry and unifies the Volume option surface with the
shared `Option` interface.

## Highlights

- Add v0.9.0 CHANGELOG entry covering CAS retry feature, unified Option
interface, and VolumeOption/WithVolumeChecksum removal
- Update V1_READINESS.md to reflect VolumeOption→Option unification
- Update compare links for [Unreleased] and new version entry

## Test plan

- [ ] CHANGELOG format follows Keep a Changelog conventions
- [ ] Compare links resolve correctly after tag is created
- [ ] No stale VolumeOption/WithVolumeChecksum references in live docs
(README, PUBLIC_API, contracts)
- [ ] `go test ./...` passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: justapithecus

CHANGELOG.md

@@ -11,6 +11,40 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ---
 
+## [0.9.0] - 2026-03-08
+
+### Added
+
+- **Opt-in CAS retry**: `WithRetryCount(n)` enables automatic bounded retry on `ErrSnapshotConflict` within the commit path for both Dataset and Volume. Data files are written once; only the manifest re-parent and pointer CAS are retried. ([#164](https://github.com/pithecene-io/lode/pull/164), closes [#163](https://github.com/pithecene-io/lode/issues/163))
+- **Backoff customization**: `WithRetryBaseDelay(d)`, `WithRetryMaxDelay(d)`, `WithRetryJitter(j)` for full control over jittered exponential backoff. Defaults: 10ms base, 2s max, full jitter.
+- **`ErrOptionNotValidForVolume`**: Sentinel error for options that do not apply to Volume construction.
+
+### Changed
+
+- **Unified `Option` interface**: `VolumeOption` function type replaced by the shared `Option` interface (already used by Dataset and DatasetReader). `NewVolume` now takes `...Option`. Options that do not apply to a given constructor return an error at construction time. ([#164](https://github.com/pithecene-io/lode/pull/164))
+- **`WithVolumeChecksum` → `WithChecksum`**: Volume block checksums now use the same `WithChecksum(c)` option as Dataset file checksums.
+
+### Breaking Changes
+
+- **`VolumeOption` type removed**: Replace `lode.VolumeOption` with `lode.Option`.
+- **`WithVolumeChecksum` removed**: Replace `lode.WithVolumeChecksum(c)` with `lode.WithChecksum(c)`.
+
+### Upgrade Notes
+
+- **Search-and-replace migration**:
+  - `lode.VolumeOption` → `lode.Option`
+  - `lode.WithVolumeChecksum(` → `lode.WithChecksum(`
+- **Retry is opt-in**: Default behavior (0 retries) is unchanged. Existing callers see no difference unless they configure `WithRetryCount`.
+- **No data migration required**: Storage layout and manifest format are unchanged.
+
+### References
+
+- [CONTRACT_WRITE_API.md](docs/contracts/CONTRACT_WRITE_API.md) — Automatic retry semantics
+- [CONTRACT_VOLUME.md](docs/contracts/CONTRACT_VOLUME.md) — Unified Option interface, Volume retry
+- [CONTRACT_ERRORS.md](docs/contracts/CONTRACT_ERRORS.md) — Automatic retry guidance for ErrSnapshotConflict
+
+---
+
 ## [0.8.0] - 2026-02-24
 
 ### Added
@@ -445,7 +479,8 @@ Post-v0.3.0 improvements planned:
 
 ---
 
-[Unreleased]: https://github.com/pithecene-io/lode/compare/v0.8.0...HEAD
+[Unreleased]: https://github.com/pithecene-io/lode/compare/v0.9.0...HEAD
+[0.9.0]: https://github.com/pithecene-io/lode/compare/v0.8.0...v0.9.0
 [0.8.0]: https://github.com/pithecene-io/lode/compare/v0.7.4...v0.8.0
 [0.7.4]: https://github.com/pithecene-io/lode/compare/v0.7.3...v0.7.4
 [0.7.3]: https://github.com/pithecene-io/lode/compare/v0.7.2...v0.7.3

docs/V1_READINESS.md

@@ -90,7 +90,7 @@ as `(private)` instead of by number.
 > Summary: —
 > Issue: #___
 
-- [ ] Option / VolumeOption surface has not expanded since dogfooding began
+- [ ] Option surface has not expanded since v0.9.0 (freeze window reset: v0.9.0 added WithRetryCount/BaseDelay/MaxDelay/Jitter, ErrOptionNotValidForVolume; removed VolumeOption, WithVolumeChecksum)
 
 > **Evidence:** _not yet recorded_
 > Date: — | Observer: — | Project: —