feat(pgpm-core): rebundlePlan — dependency-aware chunk/merge primitive with byte-identical gate

[pyramation]

Summary

Atom 4, first slice: the granularity dial between the two extremes we already have (one-file-per-change vs one giant pgpm package). rebundlePlan(moduleDir, strategy) groups a module's changes into fewer, larger chunks — but does so safely, respecting the three orderings the engine cares about (deploy topological, verify in deploy order, revert reversed).

Deliberately thin and additive — it reuses the existing plumbing rather than inventing a parallel model:

• plan parsing via parsePlanFile / getChanges
• graph + package-quotient machinery from slice/ (buildDependencyGraph, buildPackageDependencies, detectPackageCycle, computeDeployOrder, extractPackageFromPath)
• SQL assembly via the same file-concatenation resolveWithPlan already uses

Key correctness idea — contiguity ⇒ byte-identical. Chunks are contiguous runs of the source plan order, not an independent topo sort:

walk changes in plan order:
  keep appending to current chunk while (same boundary key) && (size < cap)
  a member carrying a tag seals the chunk (tags must stay deploy --to seams)

Because chunks are contiguous, concatenating chunks in order — members in source order, reverse for revert — reproduces the original resolved output exactly, at any dial position. Cross-chunk edges only ever point backward, so the chunk quotient is a DAG by construction; if a boundary ever splits a back-edge, we surface a warning instead of silently reordering.

verifyRebundleInvariant(moduleDir, result) is the gate that makes any granularity choice safe:

assemble('deploy')  === resolveWithPlan(dir, 'deploy')
assemble('revert')  === resolveWithPlan(dir, 'revert')   // chunks reversed, members reversed
assemble('verify')  === concat(verify files in plan order)

Strategy surface (all optional):

• boundary: 'folder' | 'none' (default folder, reusing slice's path extraction)
• depth / prefixToStrip for the folder key
• maxChunkSize to cap chunk size (splits a boundary group, still contiguous)

No behavior change anywhere else — new rebundle/ module exported from @pgpmjs/core, nothing consumes it yet. Actually merging chunk files / re-deparsing (the packageModule-per-chunk step) and the classifier-driven boundaries land in follow-ups; this PR establishes the chunk model + ordering + invariant.

Test plan

• 11 new tests in __tests__/rebundle/ (no DB needed): folder grouping, verify==deploy / revert==reverse, cross-chunk dep derivation, single-chunk and size-capped dials, deploy/revert assembly order, and byte-identical invariant at all three dial positions.
• tsc --noEmit and pnpm build clean.

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

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