feat(pgpm-core): rebundleModule — per-chunk packageModule merge with byte-identical gate

[pyramation]

Summary

Atom 4, second slice: turns the chunk plan from rebundlePlan (#1411) into actual merged migrations on disk. rebundleModule(sourceDir, { outputDir, ...strategy }) writes a rebundled module whose per-change files collapse to one migration per chunk, gated byte-identical against the original when packaged.

The merge reuses packageModule's exact engine rather than a second implementation. packageModule is refactored to extract that engine as mergeSqlStatements:

// shared by whole-module packaging and per-chunk rebundling
mergeSqlStatements(sql, { stripTransactions, echoExtensionName, pretty, functionDelimiter })
//   parse → filter(TransactionStmt/CreateExtensionStmt) → deparse → round-trip diff

packageModule(dir) = mergeSqlStatements(resolveWithPlan(dir), { stripTransactions: extension, echoExtensionName: extname })

Per chunk and per direction, rebundleModule merges the member scripts (transactions stripped) and re-wraps in one transaction — deploy/revert COMMIT, verify ROLLBACK — so each chunk deploys as exactly one transaction:

deploy/<chunk>.sql = BEGIN; <merge(members, deploy order)> COMMIT;
revert/<chunk>.sql = BEGIN; <merge(members, reverse order)> COMMIT;
verify/<chunk>.sql = BEGIN; <merge(members, deploy order)> ROLLBACK;

It then writes a chunk-quotient pgpm.plan (one change per chunk, deps = earlier chunk names, via generatePlanContent) and remaps tags onto their sealing chunk. The module stays the same extension — control/Makefile/package.json are copied verbatim, only the internal change set collapses.

Gate. rebundleModule returns invariant, and packaging both directories must match:

packageModule(sourceDir).sql === packageModule(outputDir).sql

This holds because merging strips the inner transactions and preserves statement order, so re-packaging strips the re-added wrappers and yields the same AST. Any deparse instability is caught by the existing round-trip diff.

Additive only; nothing calls rebundleModule yet. Follow-ups: classifier-driven boundaries (Atom 2 facts choosing seams) and the control-only cross-chunk dep mode for published module forks.

Test plan

• 6 new rebundleModule tests (real DDL, no DB): per-chunk files + chunk-quotient plan, metadata copy, single-transaction merge (one BEGIN/COMMIT; verify ROLLBACK), byte-identical gate at folder and single-chunk dial positions, overwrite guard.
• Existing packaging/modules/resolution suites: 62 tests + 29 snapshots green (confirms the packageModule extraction is behavior-preserving).
• 11 rebundlePlan tests still green; tsc --noEmit and pnpm build clean.

Link to Devin session: https://app.devin.ai/sessions/ad40d16f38a349b48eb7ce9375e27e6e
Requested by: @pyramation

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