feat(session): add turn diff route

[nexxeln]

Supersedes #47795 (same feature, clean history).

summary

• idle message (Session.Message.Idle, outcome: succeeded | failed | interrupted): projected when a busy period ends (session.execution.succeeded/failed/interrupted; shutdown is skipped because the resumed execution continues the same turn). Everything since the previous marker is one turn, steers included, so turns are derivable from session_message alone — no event persistence, no new table, no migration. Invisible to the model (toLLMMessage skips it) and to the TUI/web transcripts (skipped in reduceSessionRows and isNotice). The shared solid data layer synthesizes it live from the execution events like the other marker messages, so in-memory transcripts match the server before the next read; the UI footer anchoring is deliberately left for a follow-up.
• GET /api/session/:sessionID/diff?messageID&to&context (session.diff) → { data: FileDiff.Info[] }: the files changed by the turn containing messageID (default: the newest user message's turn); to extends the range through a later user message's turn (contiguous, commit-range style). Compares the range's first recorded step snapshot with its last recorded one; only a step still running in the actively executing session compares against the working copy. Omitted context → full-file patches like vcs.diff. Snapshot trees are resolved from the Location in effect at the range (derived from location-switched messages via LocationServiceMap.get), so pre-move turns stay diffable and a range spanning a move is rejected. Errors: 400 InvalidRequestError (to before messageID, not a user message, location change), 404 MessageNotFoundError / SessionNotFoundError, 500 UnknownError with a logged ref on snapshot failure. Shared missingMessage / failedSnapshot handler helpers replace three inlined copies each.
• Sessions with no idle marker at all (history from before this change, never resumed) fall back to prompt → next prompt. An old session that is resumed once merges its pre-marker history into the first marked turn — a superset, and transient; new sessions are always exact.
• Git.tree.diff runs --name-status, --numstat, and the patch once per tree pair instead of three spawns per changed file (~50 ms/file before), caps patch output at MAX_TOTAL_PATCH_BYTES (capped files get an empty patch, stats stay exact), uses core.quotepath=false so non-ASCII paths still match their patch chunk, and passes --no-ext-diff. Snapshot.diff diffs first and filters ignored paths from the result instead of listing changed files twice. Revert benefits too.

Backend only; no TUI/app feature work in this PR.

Note for reviewers (not changed here): packages/core/src/session/stats.ts:296 filters EventTable.type with the unversioned SessionEvent.UsageRecorded.type while bus.ts stores versioned type names (session.usage.recorded.1), so that query cannot match persisted rows.

testing

• bun typecheck in schema, protocol, core, server, client, plugin, tui, session-ui, app, cli, sdk, www.
• core: full suite 5248 pass. New test/session-diff.test.ts is one scenario with real git snapshots: legacy prompt→prompt fallback, marker-era turn with a steer inside it (default and by the steer's message), pre-marker history merging into the first marked turn, ranges via to, error cases, unrecorded-end fallback, running-step working-copy diff only when active, revert (markers included), fork. test/git.test.ts covers the patch cap and the non-ASCII header match (assertion verified to fail without core.quotepath=false). One existing recovery test now excludes idle markers from its message-list assertion.
• server: 57 pass incl. new test/session-diff.test.ts (200/400/404 contract). client 160 pass; tui data + diff-viewer 140 pass.
• protocol generate-openapi.ts --check, www check:generated + build, prettier on touched files.