feat(diff): read a diff in the order a walkthrough sets

[fiddur]

Slice 1. Tours already carried an ordered list of file-and-line stops with narrative, and the tour
skill already had a review mode that scopes them to a PR's diff — but only the repository browser
could render one, so a walkthrough of a diff had nowhere to appear.

The diff page now reads the session's most recent tour and reorders the whole view by it. Sidebar and
diff body both, because they render from one list: files the walkthrough visits come first in visit
order, everything else keeps the order it had, under a "Not in the walkthrough" divider. A toggle in
the sidebar header returns to the alphabetical tree.

• the sidebar switches to a flat numbered list in that mode — position, file, and the stop's one-line annotation — with the existing filter and comment-only controls still applied
• a stepper under the toolbar walks the stops with prev/next and scrolls to each file
• keyboard file navigation follows the reading order rather than the alphabetical one
• tours are polled on the same 2s interval as comments, and one still being written shows its steps as they arrive, so a reader watches the order being worked out
• the tour skill's hand-off now sends a review tour to /diff, and says an annotation should explain why a file is read at that point

Measured on NaturalCycles/NCBackend3#14360, 11 files:

alphabetical                                    review order
 1. src/email/email.service.test.ts             1. server/services/idempotency.service.ts   the primitive
 2. src/email/email.service.ts                  2. server/services/idempotency.service.test.ts  the contract
 3. src/sendgrid/sendgridApi.service.ts         3. sendgrid/sendgridApi.service.ts          first consumer
 4. src/server/…/idempotency.service.test.ts    4. twilio/twilioApi.service.ts              skipped vs failed
 5. src/server/…/idempotency.service.ts         5. shipping/shippingItem.service.ts         threading it through
 6. src/shipping/…/shelflessWebhook.test.ts     6. shipping/…/shelflessWebhook.handler.ts   the P1 lives here
 7. src/shipping/…/shelflessWebhook.handler.ts  ── not in the walkthrough ──
 8. src/shipping/shippingItem.service.test.ts   ·  the five signature-only test files
 …

The primitive moves from 5th to 1st, the file holding the blocking finding from 7th to last, and the
five files that are signature updates only drop below the divider.

Ordering is a pure function with tests: first-visit wins when a walkthrough returns to a file, files
it names that the diff does not contain are dropped, no file is lost or duplicated, and an empty
walkthrough is the identity. 120 UI tests pass.

Also fixes a TS2883 from #12 — tsc could not name the sanitize schema's inferred type. Note that
npm run typecheck -w @diffity/ui still reports four pre-existing TS6059 errors that are present
on upstream main too; the script is not wired into npm test.

Follow-ups worth doing: the stepper scrolls to the file, not the line, because DiffViewHandle
exposes only scrollToFile; and a step's line range is not highlighted in the diff yet.

Stacked on #12.

🤖 Generated with Claude Code

https://claude.ai/code/session_018PkYQzbsnMihHesafWvXKs