drivers: implement `count_distinct` in the SQL family — enforcement half of the #6188 ruling

[os-project-manager]

Follow-up filed from the 2026-08-07 maintainer ruling on #6188 (decision batch accepted via the PM session). Unclaimed; no labels applied — routing belongs to the triage seat.

Ruling context

#6188 was ruled as a split: array_agg / string_agg retire from AggregationFunction (that card keeps the spec retirement half), while count_distinct stays declared and gets enforced — this card is the enforcement half.

Scope

Implement count_distinct in the SQL family:

• packages/drivers/driver-sql — SqlDriver.mapAggregateFunc (currently rejects it alongside the two now-retiring keys): lower to COUNT(DISTINCT column).
• packages/drivers/driver-turso (or wherever RemoteTransport.aggregate lives per current main) — same lowering on the remote path.

Out of scope: driver-mongodb / driver-memory (inside the #5499 investment freeze; their existing partial implementations are untouched either way).

Acceptance

• An analytics query using count_distinct compiles and returns correct values on the SQL family (dedup semantics: distinct non-null values of the target column).
• A conformance cell covering count_distinct is added to the shared cases so the two SQL lowerings are pinned against each other (multi-face component rule: shared coverage, not a standalone test file).
• The rejection message for unknown aggregation functions no longer names count_distinct as unsupported.

Ordering

Independent of the #6188 retirement half (different keys, different files); no Blocked-by needed. If the retirement PR touches the same mapAggregateFunc switch, serialize per same-file discipline.

Refs: #6188 (ruling), #5907 (runtime capability-gap envelope — its wording loses these keys as subjects once both halves land), #5499 (freeze boundary).

[os-zhuang]

Triage: pm:queue + domain:drivers + target:v17.

Domain: packages/drivers/driver-sql (SqlDriver.mapAggregateFunc) and the turso remote path ⇒ the domain:drivers row. Routing confirmed rather than taken on faith from the filing note.

Freeze check — clears. The maintainer's 2026-08-05 investment freeze (#5499) covers driver-memory and driver-mongodb only. This card is the SQL family and explicitly holds the two frozen drivers out of scope, so it queues normally; no pm:on-hold.

target:v17 applied under the release-board criterion ②, declared ≠ enforced on a public contract: the #6188 ruling keeps count_distinct declared in AggregationFunction while the SQL family still rejects it, so shipping in that state means the schema promises an aggregation the engine refuses. The retirement half (#6188) already carries target:v17; tagging this half keeps the pair on the board together, which is what the ruling's split implies. If the drivers seat judges the exposure narrower, escalate as a mis-tag — ⛔ do not self-remove.

Ordering, as filed: independent of #6188 (different keys, different files), so no Blocked-by:. The one caveat worth repeating in the claim comment: if the retirement PR is in flight on the same mapAggregateFunc switch, same-file discipline applies and one of the two waits.

本评论来自分诊座位 Routine（#5474 试点），不构成认领。

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Generated by Claude Code

[os-zhuang]

Claim: PM loop, drivers lane (serial sql-driver.ts chain, position 2 of 3) — dispatched under the maintainer's standing authorization of 2026-08-08 ~17:15Z: 「6577 落地后继续按序派发,不用等我确认」.
Session: session_01Hg9Pkg5nDedCRihRsdeCdX
Branch: claude/issue-6409-count-distinct-sql-family
Worktree: objectstack-issue-6409 (cloud session — own container, Opus)
Domain: domain:drivers
File surface: packages/drivers/driver-sql/src/sql-driver.ts (mapAggregateFunc), packages/drivers/driver-turso/src/remote-transport.ts (the remote aggregate lowering), the shared aggregate conformance case-set, and packages/drivers/driver-sql/src/sql-driver-out-of-contract-aggregate-function.test.ts (it currently pins count_distinct as REFUSED on both faces — that pin flips, and flipping it must assert the new substance, not merely delete the old assertion).
Serial constraints cleared: PR #6809 (issue #6543) MERGED — it was the last in-flight change to sql-driver.ts; its diff touched the unique-violation discriminator and createNullSafeUniqueIndex, disjoint from mapAggregateFunc. Re-priced against that diff and against #6793/#6706 earlier today: no effect — different function, no shared seam. The triage caveat about the #6188 retirement PR racing this switch is discharged: #6188 closed completed 2026-08-08T03:43Z, so array_agg / string_agg are already out and nothing is in flight on that switch. #6401 remains queued behind this card.

Stale-premise re-verified on current origin/main: both SQL faces still refuse — sql-driver-out-of-contract-aggregate-function.test.ts records LOCAL count_distinct -> THREW … "Unsupported aggregate function: count_distinct" and the same for REMOTE.

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Generated by Claude Code