refactor(migrations): split the registry into per-topic files

[krlmlr]

Preparatory PR for the repo-wide ellipsis migration (#2757):
a single commit with the registry-split logic plus the split of the
existing migrations, so every follow-up PR can carry it as its first commit
and rebase to a pure diff once this lands.

What this does

• Per-topic registry files. tools/generate-migrations.R now loads every
  tools/migrations/<topic>.R file (one per topic, mirroring the R sources),
  merges the entries, and rejects duplicate declarations across files.
  A legacy tools/migrations.R is still honoured if present, easing older
  branches (e.g. refactor: snake_case attr-comb options & arguments, soft-deprecate dotted names #2742) over the transition.
• Existing migrations move as-is: as_adjacency_matrix and
  as_biadjacency_matrix to tools/migrations/conversion.R, the test fixture
  to tools/migrations/fixture.R. The spliced ARG_HANDLE blocks are
  byte-identical (the generator run is a no-op on R/).
• tools/migrations.R is deleted; its schema documentation moves to
  tools/migrations/README.md.
• Two generator fixes (both found while applying the pattern at scale,
  each with a regression test):
  • a single over-long item rendered as list(, item) — syntactically valid
    but an error the moment the block runs — because paste0() turns
    character(0) into ",";
  • deparse() emits x/y where air formats x / y, failing CI's
    idempotency check for any block whose defaults contain a division;
    spaces are reinserted token-aware via getParseData(), so slashes inside
    string literals (URLs) stay untouched.
• New unit tests for the registry loader
  (tests/testthat/test-generate-migrations.R): multi-file merging, duplicate
  rejection, empty registries, plus the two regression tests.
• tests/testthat/helper-migrations.R, the in-sync test in
  test-migration-fixture.R, and the CI drift check
  (.github/workflows/custom/after-install) now resolve the registry via
  migration_registry_files() instead of the hard-coded single file.

Follow-ups (held until this merges)

• docs: zoning rules for arguments around the ellipsis #2757 rebases onto this and becomes docs-only (the zoning rationale in
  CONTRIBUTING.md).
• The 21 topic PRs (refactor: move optional arguments of trees functions behind the ellipsis #2758–refactor: move optional arguments of misc functions behind the ellipsis #2778) get rebased to [this commit, topic commit]
  and force-pushed.

Note

#2742 appends entries to the deleted tools/migrations.R and will conflict
trivially: its three entries just move into a topic file (the legacy file also
still loads if resurrected, so an interim merge is harmless either way).

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