[docs] docs/experiments/README.md: six records unindexed, and no 2026-09 section

[eaitbrahim]

docs/experiments/README.md says it is the index of this directory, "newest first". It is not current:

• Six records are absent entirely — 2026-09-01-cusum-event-first-measurement.md, 2026-09-01-triple-barrier-first-measurement.md, 2026-09-02-equities-cost-fidelity.md, 2026-09-03-equities-dca-benchmark.md, 2026-08-27-pooled-review-preview.md, and there is no ## 2026-09 section at all.
• The file's own preamble makes the index a claim about the directory: "Index is newest first, by the date each document carries in its filename." An index missing a quarter of its records is worse than none, because it reads as complete.

This matters more here than in an ordinary docs directory: the README is the entry point a reader uses to find what has been measured, and three of the missing records are the ones that supersede figures other documents still carry.

Done when

• Every .md in docs/experiments/ has an index entry
• Every link in the index resolves to a file that exists
• Committed .py drivers and .jsonl artifacts are named beside the record that cites them, per the directory's stated convention