feat(session): add turn diff route

[nexxeln]

summary

• GET /api/session/:sessionID/diff?messageID&to&context (session.diff) → { data: FileDiff.Info[] }: the files changed by a turn, computed on request from the per-step snapshots already stored on assistant messages. Nothing new is persisted.
• A turn is the assistant steps that follow a user message up to the next user message (a steer is a new prompt, so it starts a new turn). messageID defaults to the newest user message; to extends the range through a later user message's turn, so a client can show "all my steers" or a commit-range-like selection. Fully derivable from session_message, so forks and reverts need no special handling.
• Compares the range's first recorded start snapshot with its last recorded end snapshot; only a step still running in the actively executing session compares against the working copy, so a crashed turn never diffs against later manual edits. Omitted context → full-file patches like vcs.diff.
• Snapshot trees live in the repository of the Location that captured them: the range's Location is derived from location-switched messages and resolved through LocationServiceMap.get, so pre-move turns stay diffable; 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 failedSnapshot helper, also used by the revert handlers; missingMessage replaces three inlined mappings).
• Second area: Git.tree.diff runs --name-status, --numstat, and the patch once per tree pair instead of three spawns per changed file, 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 --no-ext-diff. Snapshot.diff diffs first and filters ignored paths from the result instead of listing changed files twice.
• Backend only. History note: this branch first tried vcs.diff?mode=turn and then an idle-based turn model with a session_turn projection; both were reverted after discussion in favour of the per-user-message definition, so the intermediate commits net to zero and the PR should be squash-merged.

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, cli, app, sdk, www.
• core test/session-diff.test.ts (one scenario: default/last turn, explicit turn, ranges, steer as its own turn, to ordering and non-user-message errors, unknown message, unrecorded end fallback, running-step working-copy diff only when active, revert truncation, fork) and test/git.test.ts (patch cap + non-ASCII header match; verified the assertion fails without core.quotepath=false); plus snapshot/revert/projector/compaction/prompt suites — 97 pass.
• server test/session-diff.test.ts (200/400/404 contract on the default non-persisting bus); full server suite 57 pass. client 160 pass; schema 61 pass + the pre-existing contract-hygiene "limits Any" failure.
• protocol generate-openapi.ts --check, core migration --check, www check:generated + build, prettier on touched files.

[nexxeln]

Superseded by #47821: same feature reworked per the design discussion (turn = between idle states, recorded as an idle message instead of a table; GET /session/:id/diff?messageID&to), on a clean branch.