Project board's Task completion median is always —: the metric waits for a status and a stamp the store never writes #1884

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment.
Written-by: FlyBot Lead

Defect

The Project board's Task completion median (taskDoneMedianMs, the —/number under "Task completion") is always — on a real deployment, because doneTaskDurations waits for a status and a stamp that no host path ever produces.

• core/src/host/project-metrics.js:56-70 (on 33e9c6d5) keeps a task only when task.status === 'done' and Number.isFinite(task.completedAt).
• A card finishes through transition(task, 'accepted') (core/src/host/tasks.js:269-279), which writes only status, updatedAt and an entry on actors ({ status, at }); TRANSITIONS ends at accepted / cancelled / failed, so 'done' is never a record status.
• completedAt is read in project-metrics.js and written nowhere in the repository — a card that reached the board's Done column has no such field.
• core/src/shared/task-board.js:8-19 maps accepted / cancelled / failed to the done column; a literal 'done' status is not in that table and would paint as not-started.
• docs/subsystems/core-host.md:793 promises fields the host never writes (reads createdAt / completedAt from state.tasks).

Existing tests (core/test/project-metrics.test.js:119-121, core/test/project-metrics-scope.test.js:43,46) hand-write status: 'done' plus completedAt, which is exactly the false-green shape: the fixtures assert a record the store cannot produce.

Reproduction (clean 33e9c6d5, sources unchanged, only core/test/project-metrics.test.js taken from the fix)

A card in the shape the store really writes — status: 'accepted', createdAt 10h ago, actors: [open@-10h, claimed@-6h, accepted@-1h], no completedAt — must contribute 9h (32400000). It contributes nothing:

$ TZ=UTC node --test test/project-metrics.test.js
not ok 10 - the task median reads the store stamps only
    null !== 7200000
not ok 13 - a card that finished the way the host finishes cards is a done duration
    null !== 32400000
# tests 14 / # pass 12 / # fail 2

The two null guards pass on the base by design — nothing is counted at all today, so they only start guarding the accepted-only rule once the first line turns green.

Expected behaviour

One rule, read from the board's own column table — no second status table, no invented state, no back-filled completedAt:

• a card counts when columnOf(task) === 'done' and task.status === 'accepted'; cancelled and failed are in the done column but never completed, so they are skipped (the card's caption is "created to done");
• its duration is createdAt → the at of the last terminal entry on actors, falling back to completedAt (finite) and then updatedAt for records without actors;
• the seven-day window still compares that end instant; an empty window or a board whose only done-column cards were cancelled/failed answers null (the client's —), never 0;
• docs line core-host.md:793 states the real source.

Scope

• core/src/host/project-metrics.js and its mirror patches/ds-bot/files/src/host/project-metrics.js
• the red-first cases themselves, inside core/test/project-metrics.test.js (and its mirror)
• fixture corrections in core/test/project-metrics.test.js and core/test/project-metrics-scope.test.js (the status: 'done' fixtures above, changed to the status the host writes) plus their mirrors
• docs/subsystems/core-host.md:793, one changelog fragment; generate + check three lines stay green

No client change, no shared/task-board.js change, no state-machine change.

Dedupe

search/issues on live main: taskDoneMedianMs → only #1880 and #1848 (both closed; they scope the same metric to one board), completedAt → 0 hits. This defect is a different one: the metric waits for a status and a stamp the store never writes.