[thekevinbot]

Fixes #1483

Cuts the serial bookkeeping in the config-table build. Before this change, dirsql's CPU time on the frontmatter case was already about the same as find + xargs -P. The gap came from work done one file at a time on a single thread.

• The scanner returns Found { path, rel_path, table_name }. Each walk worker builds the relative path for the files it matches (found_in), so the calling thread no longer recomputes it for every file. relative_path is removed, and reconcile_scan no longer needs the anchors.
• finish_build_with_fs groups the scanned files on a scoped thread (group_scanned) while the main thread inserts rows from the per-table hooks that started early (stream_early). Before, grouping ran first and the inserts waited behind it. Grouping also checks the last batch before searching the list.

Not on the vtab/output path that #1513 changes.

Measurements

Corpus: 131072 notes. Paired, alternating runs; median ratio vs find+xargs-P. The machine was shared with other agents, load 11-27.

	before	after
ab.py, 21 pairs, wall time	204ms, ratio 2.10	169ms, ratio 1.81
frontmatter_parse_speed_test (startup subtracted), ratio	2.4-3.2 (issue)	1.91

The test's bar native (find+awk) is unchanged, and the test still passes against it (ratio 0.185). It is not switched to bar=FASTEST, because that would leave the test red at 1.9x.

Changelog / Migrations

• Changelog fragment added under packages/rust/changelog.d/
• Migration fragment with the same slug added under packages/rust/migrations.d/

E2E Verification

• Ran e2e suites locally for every affected SDK
• Python SDK e2e: pass (impacted subset, 22 passed)
• TypeScript SDK e2e: pass (18 passed)
• Rust core e2e (if applicable): pass (cargo test --workspace --features cli)
• packages/python/e2e-attestations/bot-1483-fm-speed-1015.json receipt written
• packages/ts/e2e-attestations/bot-1483-fm-speed-1015.json receipt written
• Command(s) run: testing-conventions e2e attest 'uv run python -m pytest tests/e2e/frontmatter_parse_speed_test.py tests/e2e/jsonl_parse_speed_test.py tests/e2e/on_file_query_test.py tests/e2e/fanout_test.py tests/e2e/declared_table_name_test.py tests/e2e/query_contract_test.py tests/e2e/use_case_notebook_test.py tests/e2e/use_case_transcripts_test.py tests/e2e/sigint_interrupt_test.py -x -q -s'; just e2e-attest-ts
• Result summary: all pass. Speed ratios against the fastest native: frontmatter 1.91, jsonl 1.31, notebook 0.91, transcripts 0.63.