feat(merge): MSSQL OUTPUT clause on MERGE / INSERT / UPDATE / DELETE

[productdevbook]

Summary

• Wires up SQL Server's OUTPUT clause through the same MergeNode.returning slot the PG path uses (PR feat(merge): RETURNING on MERGE (PG 17+) #148 set up RETURNING on MERGE for PG 17+; MSSQL previously threw via MERGE_RETURNING feature flag). The MSSQL printer now rewrites the tail to OUTPUT <projections> instead.
• Adds mergeActionMssql() builder helper for SQL Server's $action pseudo-column — kept distinct from PG's existing mergeAction() (which compiles to MERGE_ACTION()) because the two emit different SQL shapes (function call vs. pseudo-column). Documented the dialect-divergence in JSDoc on both helpers.
• MERGE_RETURNING feature flag now includes mssql; the existing INSERT / UPDATE / DELETE OUTPUT paths got a small extension to respect explicit INSERTED / DELETED table qualifiers on column refs (so UPDATE OUTPUT INSERTED.id, DELETED.id works without the auto-prefix double-wrapping the user's choice).

Pseudo-table conventions on MERGE

• Bare col("id") → INSERTED.[id] (post-action row — the common case).
• Explicit col("email", "DELETED") → [DELETED].[email] (pre-action row).
• star() → INSERTED.*; pass star("DELETED") for the deleted set.
• mergeActionMssql() → bare $action token (no parens — it's a pseudo-column).

Implementation notes

• The $action token can't be a function call (validateFunctionName rejects $ prefixes) and a raw node would be flagged by the findRawNodes audit. We use a sentinel function name (__SUMAK_MSSQL_ACTION__) that the MSSQL printer recognizes and rewrites to $action; on other dialects the sentinel falls through and the engine errors at parse pointing at the offending call site (the desired "wrong but visible" behavior).
• The MSSQL printer's printMerge re-uses the base printer's MERGE skeleton (target / source / ON / WHENs / CTEs) and only adds the OUTPUT <cols> tail — no duplication of MERGE-statement logic.
• Cut: OUTPUT … INTO @var (writing to a table variable) — sumak doesn't model variable declarations.

Test plan

• MERGE … OUTPUT $action, INSERTED.[id] for the printer surface (both untyped and typed builder).
• PG path unchanged: RETURNING MERGE_ACTION(), "id", "name" still emits the same way; existing PGlite roundtrip passes.
• INSERT / UPDATE / DELETE OUTPUT paths cover the mixed INSERTED + DELETED qualifier case (UPDATE projecting both pre- and post-image).
• pnpm fmt && pnpm lint && pnpm typecheck && pnpm vitest run all green (2076 tests pass, no new lint warnings).

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