Drive pdm.lock and poetry.lock rewrites through one lock_fragments driver instead of two mirrored triads #1844

[agent] Filed by the scheduled architecture audit (audit-core). Register: register.

Kind: refactor. Register ID: A05.

Problem

utils/pdm_lock.rs and utils/poetry_lock.rs each carry the same driver around utils::lock_fragments:

• Poetry has rewrite_poetry_lock / _with_edits / _in / _all / _all_with_version / _batch / _steps: poetry_lock.rs L426-L731.
• PDM has the mirror: pdm_lock.rs L136-L353.
• The test modules are mirrored too. parse_reuse_equivalence_tests and batch_equivalence_tests exist in both files, and the "fragments changed shape" test exists three times (lock_fragments L595, pdm_lock L921, poetry_lock).

The _steps loops have already drifted:

• PDM calls rewrite.edits()? before its Unchanged check: pdm_lock.rs L333-L340.
• Poetry compares the text first: poetry_lock.rs L710-L719.
• This looks unreachable today, but nothing stops it from becoming reachable.

Inside lock_fragments itself, fragment pairing is written three times, and the copies disagree on how uniqueness is counted:

• pair_fragments L191-L210 counts non-overlapping matches on both sides.
• the finish loop L316-L336 checks old in the text and new in the spliced result.
• rewrite_batch L473-L495 counts overlapping sole_occurrences.
• The shape-change length check is repeated at L198, L322 and L477.

Proposed change

• Move the _steps, _all and _with_edits drivers into lock_fragments, generic over a small per-format value: plan, mutate, fragments_in.
• Delete the per-format wrappers and the PoetryLockRewrite/PoetryLockParse aliases.
• Have finish and rewrite_batch call one pairing helper.
• Keep one copy of each equivalence test, parameterized over both formats.

Size

Three files (lock_fragments.rs, pdm_lock.rs, poetry_lock.rs), plus call-site renames in redirect/pdm.rs, redirect/poetry.rs, vendor/pypi_pdm.rs and vendor/pypi_poetry.rs. About −250 net lines.

Acceptance criteria

• Every pdm/poetry exact-bytes test passes unchanged, including the batch and parse-reuse equivalence tests (kept once, run for both formats).
• The duplicated shape-change test is kept once, in lock_fragments.
• No user-visible change.