feat(frontend): model conversation turn narration with Statig

[sandersonstabo]

Depends on #349.

Scope:

• add the Rust Statig per-turn narration state machine
• model provider wait, compaction, thinking, work, streaming suppression, background wait, Worked/Thought terminal copy, interruption, failure, and cancellation
• enforce monotonic timestamps/revisions, resumable interruption, sealed terminal idempotence, and a single renderer-facing narration
• register focused Bazel/Cargo coverage

Excludes:

• no TypeScript, Svelte, or JavaScript changes
• no scene composition, GPUI rendering, or host wiring (stacked follow-up leaves)

Verification:

• guarded jobs=1 focused build and behavior test pass
• fresh production and test Clippy pass
• whole-repository Rust format gate passes
• locked/offline Cargo metadata passes
• guarded hashes/status stable; Bazel shutdown exit 0

Evidence: D:/artisan-editor-worker-logs/frontend-conversation-turn-vp-gate-20260830/forward4/receipt.json

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Summary by cubic

Introduces a Statig state machine that models conversation turn narration in Rust, replacing scattered thinking/compacting/streaming booleans with a single authoritative chart that derives exactly one renderer label per state.

• Covers provider wait, compaction, thinking, work, streaming suppression, background wait, and the terminal Worked/Thought/Failed/Stopped/Cancelled labels.
• Enforces monotonic timestamps and revisions, rejects new events on sealed terminal states, and allows only Interrupted to resume.
• Adds a ConversationTurnController per turn id with a closed TurnNarration enum so two simultaneous labels are unrepresentable.
• Registers focused Bazel/Cargo coverage and adds black-box behavior tests for narration, atomic refusals, idempotent duplicates, and extreme elapsed arithmetic.
• Includes no TypeScript, Svelte, GPUI, or host wiring; that remains for stacked follow-up leaves.

^{Written for commit 36ac01d. Summary will update on new commits.}