feat(propose): --table-grid selects Arm E, absent is off [skip-docs]

The flag threads through `run` and `main` and takes no argument. Arm D's gate
is a run LENGTH where 0 means off; Arm E has no numeric parameter, so a boolean
is the honest shape and an integer would only manufacture a sweepable knob that
means nothing. `run` therefore adds no numeric validation, and the help says
why.

Both prose sites that enumerate the arms `okf build` does not expose are
updated: `src/llm_ingestion_okf/cli.py` and `CLAUDE.md`. The second was found
by review, not by grep of the first -- the same claim lives in two files and
only one of them is code.

The generalised attribution test earned itself in this commit. The first draft
of the Arm E help contained "byte-identical to Arm D -- Arm D rather than Arm
B", and argparse's rendering plus the test's ` --` chunk split meant the
attribution fell OUTSIDE the `table-grid` chunk. The test went red with the
truncated chunk printed, which is exactly the failure it exists to catch: a
whole-output grep would have been satisfied and the attribution would have been
unfindable in the option it belongs to. The clause is now parenthesised.

Arm C's marker check in `tests/test_cli_build.py` gains `rule:table-grid` and
is renamed to speak of all three arms, measured on the artifact rather than on
the flag: a flag `okf build` never passes is not evidence about what it emits.

[skip-docs] is the MEASURED precedent, not a convenience. `grep -c` for
"outline-run", "max-segment-chars", "Arm C" and "Arm D" returns 0 in both
README.md and CHANGELOG.md: an arm flag is documented in its constant's `#:`
comment, in `--help`, and in the round's measurement report, and it is off by
default so it makes no promise to a consumer. `--path-prefix`, which is a real
interface change, does have a CHANGELOG entry. The rule this follows is stated
at docs/2026-09-07-k3-arm-d.md: "interface and behaviour changes yes, arm flags
no." Arm E's report is docs/2026-09-07-k3-arm-e.md, later in this round.

Tests first: 3 red, then green (a fourth, the no-argument test, is honest in
its docstring that it is green before the flag exists too, because argparse
rejects an unknown option with the same code; it becomes evidence only once
the flag is real). 1248 -> 1251.
ruff check: exit 0. ruff format --check: exit 0. mypy --strict src/ tools/:
27 files, Success. pytest -q: exit 0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -245,8 +245,8 @@ and fixtures, never code.
   recursively, and two same-named documents in different folders must not
   collide); `--ingested-at` and `--proposed-at` default to one shared epoch
   constant rather than the clock, because a wall-clock default takes
-  rebuild-equals-incremental away from anyone who omits them. Arm C and
-  Arm D are off and not exposed here.
+  rebuild-equals-incremental away from anyone who omits them. Arm C, Arm D
+  and Arm E are off and not exposed here.
 - Consume a bundle: `python3 tools/okf_consume.py <bundle> --question "<q>"
   [--k N] [--limit N] [--out PATH] [--ref IDENTITY]` — the **pre-pass**
   `docs/consumption-contract.md` § 1 defines, and the only reading direction