chore(ariba): retire ariba_batch.py, unused by the capture path (#186)

[alexwolson]

Closes #186.

What this removes

ariba_batch.py implemented the batched-bundle capture: split an oversized picker selection into
sub-500 MB groups, download each as a zip, merge them. capture_event has gone through
ariba_files.capture_files since #174 (per-file download, canonical zip built locally), so
nothing on the live capture path has called it since.

It was deliberately left in place at the time — "deleting the old mechanism in the same change
that introduces its replacement is how you end up with neither." Per-file capture has since
archived a real event end to end (37 documents, 178 leaves, valid zip, #182), so that condition
is met.

The checklist, verified before deleting

The issue asked to confirm three inherited lessons live in ariba_files.py independently before
the file goes:

• Atomic bundle write — .tmp + os.replace, in build_bundle. ✅
• A corrupt/missing manifest discards partials rather than adopting them positionally —
  capture_files's manifest is None branch (shutil.rmtree + restart). ✅
• The gap record is written before the bundle, never after — write_omitted → build_bundle
  → clear_omitted_when_complete, in that order. ✅

ariba_batch.py's own batching-specific lesson — a batch's identity is its on-disk sidecar,
never an in-memory count — does not port over: per-file capture builds one canonical zip
directly from individually-downloaded files, with no batch-splitting/merging step to have that
identity problem in the first place. Confirmed by reading the module before deleting it, not
assumed.

What's NOT touched here

The issue also asked to check AribaPicker, noting it "is shared with expected_count(), so it
stays regardless (and see #185, which may remove that caller too)." #185 has since landed and did
remove that caller — AribaPicker and _select_all_attachments are unused by the live capture
path now, same as ariba_batch.py was. Per your explicit request in the #185 PR discussion, that
retirement is tracked separately as #195, not folded into this change. Both are left exactly
as #185 left them.

Cleanup

Removed sources/ariba_batch.py and tests/test_ariba_batch.py together (929 lines, ~63 tests).
Updated the historical comments referencing ariba_batch across ariba_files.py,
ariba_attachments.py, and CLAUDE.md — past tense, with a #186 pointer — rather than leaving
them describe a module that no longer exists as though it still does.

Verification

• Confirmed via grep that no import of ariba_batch remains anywhere in toronto_bids/ or
  tests/, and confirmed via direct importlib that the module is genuinely gone.
• 781 tests passing (844 − 63 deleted with the retired module).

🤖 Generated with Claude Code

https://claude.ai/code/session_01W53WHx8mm2UHuLFAQWeF62