feat(database): read bounded conversation patch replay

[sandersonstabo]

Depends on #321.

Scope

• add a one-transaction, per-thread durable conversation patch replay reader
• return typed Current, bounded contiguous Batch (max 64), or ResnapshotRequired outcomes
• reconstruct all five domain patch variants with the accepted E2/E5 timestamp and revision mapping
• reject gaps, malformed durable payloads, and content-leaking corruption paths
• add 12 focused database tests for bounds, cursors, all variants, isolation, gaps, and corruption defense

Verification

• focused Bazel database/test build with jobs=1
• //tests/database:conversation_patch_replay_test (all 12 cases)
• fresh production and integration-test Clippy markers
• //:format_test
• cargo metadata --locked --offline --no-deps
• exact eight-path hash/status/diff guards and Bazel shutdown 0

Exclusions / remaining work

• no backend RequestHandler subscribe/unsubscribe wiring
• no SessionDelivery publication loop or notifier
• no transport/client/frontend changes
• no startup/custody/provider policy

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Summary by cubic

Adds a bounded, transactionally consistent conversation patch replay reader to the database module. The reader returns typed Current, contiguous Batch (max 64), or ResnapshotRequired outcomes, reconstructs all five patch variants, and rejects gaps and malformed payloads.

• Corruption errors use fixed strings instead of interpolated persisted values so they cannot leak patch content.
• Adds 12 database tests covering bounds, cursors, variants, isolation, gaps, corruption, and error-content leakage.
• Registers the reader and its test target in Bazel and Cargo.
• Not yet wired to the backend request handler, session delivery, transport, or frontend.

^{Written for commit f945d8e. Summary will update on new commits.}