docs(experiments): every flat-priced record points at the cost correction

[eaitbrahim]

Recommended by #335's own restatement, not done at the time.

Sixteen records report profit factors priced at the flat 5bp slippage floor. One document — the restatement itself — knows that no asset in keel's universe reaches that floor (1.1× to 36.8×, median ~10×).

Why a pointer and not a re-run

The correction is conservative-only: real cost is higher, so a corrected profit factor can only fall, and every one of the 120 measured deltas was negative or zero. Re-running thirteen documents to move numbers that were already null spends real compute to reach the same conclusion.

What changes isn't any document's verdict — it's what a reader may conclude by comparing across documents. A 0.9 in one and a 1.06 in another aren't on the same scale when both were priced at a rate neither asset trades at, and nothing on the page said so.

Nothing is rewritten

docs/experiments is an append-only account of what was run; editing a measured number would falsify it. Every change here is an addition — git diff origin/main -- docs/experiments/ contains no removed line that isn't part of a note being reflowed.

The index needed its own note, because the blanket one was false there

Applied uniformly, the note told docs/experiments/README.md that "the figures below are priced at the flat floor" — and the index has no figures. It now says what's true of it: that every record dated before 2026-09-01 is flat-priced.

A note that misdescribes the page it sits on is worse than no note. I caught this by reading the rendered output rather than trusting that a uniform edit was uniformly correct.

Test first

The third assertion is the one worth having: a bare link would read as a retraction, which is a worse error than the one being corrected. The pin requires the word "verdict" within 600 characters of the link, and a mutation replacing the qualification with "These numbers are wrong." fails it.

Tests: tests/test_experiment_cost_pointers.py — 3 tests.
3 mutants, 3 killed: a record losing its pointer; a pointer that reads as a retraction; and the discovery regex matching nothing — that last one because every assertion here passes vacuously if the scan finds no records.

Full suite 5258 passed / 3 skipped; ruff clean.