feat(backend): stream conversation patch delivery

[sandersonstabo]

Depends on #331.

Scope

• add the backend-owned lazy unidirectional conversation delivery writer
• send the first real PatchBatch frame before advancing the subscription registrar
• reuse one stream across contiguous batches and reset unfinished ownership on failure/cancellation
• preserve typed open, send, registry, and finish failures
• register the focused real-loopback proof in Cargo and Bazel

Exclusions

• no queue, notifier task, connection-driver integration, listener/startup composition, frontend routing, or Event frames

Verification

• exact-head jobs=1 backend leaf gate passed at 55fdb03fce8162459d1d36011a55538c21e014f6
• focused build passed
• 8/8 real-loopback Rust cases passed
• fresh production and test Clippy markers passed
• root format and locked/offline Cargo metadata passed
• exact seven-path hash/status guards and Bazel shutdown passed

Gate receipt: D:/artisan-editor-gate-logs/backend-conversation-delivery-writer-20260829-gate2/receipt.json.

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Summary by cubic

Adds a backend-owned lazy conversation delivery writer that sends the first real PatchBatch frame before advancing the subscription registrar, reusing one stream across contiguous batches and resetting unfinished ownership on failure or cancellation.

• Opens a server uni stream only for real replay batches; Current and ResnapshotRequired outcomes return without touching the wire or registrar.
• Advances the registrar only after the batch envelope is sent; open, send, registry, and finish failures preserve their typed sources and consume the writer.
• Resets a dropped unfinished stream with a fixed application code.
• Adds 8 real-loopback tests covering publish, reuse, send failure, registry failure, and cancellation.

Exclusions

• No queue, notifier task, connection-driver integration, listener/startup composition, frontend routing, or Event frames.

^{Written for commit 55fdb03. Summary will update on new commits.}