The story opens wave to changes to pulls to conversation, one tap each, and a test walks it #10702

Intent: the changelog is read progressively: a wave, its changes, a change's pull, the pull's conversation, never losing the place.

Done:

• [x] Starting at the newest wave (src/story/waves.ts), each level links to the next and back to its parent with the wave's anchor kept in the address.
• [x] A test fetches a seeded wave page, follows the first change link, then its pull link, then the conversation, asserting 200 at each and that the back link returns to the same wave.
• [x] Each page answers under 50 ms warm (Server-Timing asserted loosely, as test/milestones-cache.test.ts does).
• [x] A level with nothing below it shows no empty link.

Files: src/story/waves.ts, src/web/pages/changelog.ts, test/story-walk.test.ts. Risks: if a link already exists, the work is the test and the missing back link only. Brief: a Story builder (scout).

Reviewed pull #10706 at its head: ticked Starting at the newest wave (src/story/waves.ts), each level links to the next and back to its parent with the wave's anchor kept in the address.; A test fetches a seeded wave page, follows the first change link, then its pull link, then the conversation, asserting 200 at each and that the back link returns to the same wave.; Each page answers under 50 ms warm (Server-Timing asserted loosely, as test/milestones-cache.test.ts does).; A level with nothing below it shows no empty link..