ci: run SQL suite over subprocess + http + unix transports

[rustyconover]

Adds multi-transport (subprocess + http + unix) coverage for the haybarn SQL
suite, with the full suite GREEN over all three transports.

What changed

• ci/run-integration.sh is now parameterized by TRANSPORT
  (subprocess default, plus http and unix). The offline mocking
  (VGI_GOOGLE_MOCK=1 + a temp VGI_GOOGLE_DISCOVERY_DIR built from
  tests.mock_google) is exported by the script and inherited by the worker
  for all three transports — no separate mock server. For http it injects
  INSTALL httpfs FROM core; LOAD httpfs; into the staged tests and boots the
  worker with --http --port 0 --port-file <f>; for unix it boots with
  --unix <sock>. One cleanup() trap captures $? first, then removes the
  discovery dir and kills the worker. A silent-skip guard (tee +
  PIPESTATUS) fails the leg unless the runner reports All tests passed (N>0 assertions …) and nothing was skipped.
• pyproject.toml: new http extra (vgi-python[http], waitress).
• google_worker.py: PEP 723 header depends on vgi-python[http].
• uv.lock: relocked with the http extra (uv sync --frozen --extra http clean).
• .github/workflows/ci.yml: the integration job is now a
  transport: [subprocess, http, unix] × os: [ubuntu, macos] matrix; sets
  WORKER_CMD, runs uv sync --frozen --extra http. Test + lint jobs unchanged.
• ci/README.md: documents the transport matrix, offline mocking across all
  transports, httpfs-on-http, the silent-skip guard, and the externalized
  pagination cursor.

Worker

No worker code change needed: the worker's externalized _ScanState cursor
(Google's nextPageToken carried in an ArrowSerializableDataclass) already
round-trips correctly over the stateless HTTP transport, so the centerpiece
pageToken page-boundary round-trip (google_drive(count := 5, page_size := 1)
→ five distinct rows) passes over http unchanged.

Local validation

• subprocess: GREEN — 31 assertions
• http: GREEN — 33 assertions (31 + injected httpfs INSTALL/LOAD)
• unix: GREEN — 31 assertions

🤖 Generated with Claude Code