feat(lab): per-file grep budget + coding_markdown_v5 arm

head_limit was a single corpus-global row accumulator spent in document
order, so one verbose early file drained the whole allowance and every
later file emitted nothing. Measured on the 2026-08-06 CoC run: the model
worked around the starvation by re-issuing one fixed pattern 14 times, one
path= per file, across 4 rounds - 469.5k logical input where the same bytes
in one round cost 258.2k. Round count is the multiplier (confirmed twice:
opus-5 did 32 calls in the same 8 rounds for the same logical input), so
fair allocation here is a round-trip lever, not formatting.

MIKE_GREP_PER_FILE_BUDGET renders each file into its own bucket and splits
head_limit max-min fair after the sweep, once the matching-file count is
known: files wanting less than an even share release the surplus, so a
corpus where only three files match still spends the full budget on those
three. The corpus sweep is never cut short, and truncated buckets are
trimmed back to their last real content row so a dangling section-lead
never reads as a match with no body.

The v5 Grep description states the new semantics - a model that still reads
head_limit as a first-come cap keeps paying for the per-path greps the
mechanism removes. v5 = v4 + this one flag; system-prompt sha is identical
to v4 and only tool_schema_sha256 moves, so the contrast has one lever.
Frozen arms clear the flag explicitly and assert it in conformance.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pfUVhNFTRvhYGXBwoKNj6