docs: move round-by-round parity narrative out of README into CHANGELOG and separate reproducible vs closed-set numbers

[luisleo526]

Problem

README.md currently carries the full round-39 write-up inline (from line 128: scoreboard, per-probe fix narrative, "12,408 physical trade pairs", "704 hard-surface probes", etc.). This reads as a changelog entry, not as project documentation, and it will keep growing each round.

Separately, the headline badge and first paragraph present 4,190 / 4,190 probes as the parity figure, but only the public board is reproducible by a reader:

Board	Probes	Reproducible from this repo + submodules
Public corpus	309 graded (312 strategies)	Yes
Closed test set	3,881	No (community scripts, not redistributed)

The README does disclose this in the scoreboard table, but the badge, the intro bullet and the "Proven, not promised" heading lead with the combined number.

Proposal

1. Create CHANGELOG.md (Keep-a-Changelog style). Move every "Round N" section and the per-probe fix narratives there, keyed by version tag.
2. In README.md, keep a single compact scoreboard table plus a one-line pointer to the changelog.
3. Make the public number primary wherever a reader can verify it: badge text like public corpus 309/309 · closed set 3,881/3,881, or two badges. State explicitly next to the closed-set figure that it is maintainer-attested only.
4. Add a short "How to reproduce the public figure" block (three commands: init submodules, scripts/run_corpus.sh, read the report) near the top, since that is the strongest claim an outside reader can check.

Acceptance

• README has no section that changes every round.
• The first number a reader sees is one they can reproduce locally.