0545 — The waterline worker is renamed cursor_advancer; AdvanceWaterline becomes AdvanceCommitted

Context

“Waterline” was design-round vocabulary that leaked into code: a whole package (pkg/worker/waterline), a persisted worker name, a controller verb, log lines, and ~150 comments. The fact it names is stored in the neutrally-named cursor.committed column ([0141]), and [0123] already settled that the codebase’s own nouns are the naming authority.

Decision

• pkg/worker/waterline -> pkg/worker/cursoradvancer; every symbol follows (CursorAdvancerDefinition/Instance/Controller/Datastore, WorkerCursorAdvancer = "cursor_advancer").
• The verb is AdvanceCommitted(topicId, groupId) (int64, error) — “committed” is the object advanced, so it stays in the name; the two-statement advance and returned committed are unchanged ([0537]).
• Config.RollRetry -> AdvanceRetry; the instance’s tick method is advance; “roll”/“rollup”/“waterline” wording is rewritten onto “committed advances” in comments, labs, CLI help, and the justfile.
• Out of scope: reference/waterline (frozen separate module), bench/scale, doc history, and decision records — append-only prose keeps the old word.

Consequences

• The persisted worker.name value changed; pre-v1 drop+recreate covers it, and workerclaimlab asserts the new name.
• Alternatives rejected: committer (collides conceptually with the cursor path’s Commit and coins an agent noun); bare Advance on the controller (drops the object).