Docs: mark M4 complete, and move the "what it turned out to mean" out of the roadmap

[kjpatel]

The roadmap had become a changelog. This marks M4 complete and moves the accumulated detail somewhere it belongs.

The drift, measured

Milestone	Words
M0–M3 (complete)	46, 132, 151, 210
M4 (before)	730
M4 (after)	189

The shape of it is legible inside M4 itself: the four bullets ticked before this milestone's habit took hold run 10–19 words; the five ticked during it run 50–174. A plan that records outcomes stops being scannable as a plan — which is the job the roadmap is the only document doing.

The v1.2 what-if bullet got the same treatment: 143 → 37 words.

v1.3+ is still 503 words across three bullets, but that is unshipped work where the length is design rationale rather than accumulated results. Different problem; left alone.

The detail is moved, not deleted

New CHANGELOG.md carries what each release turned out to mean — the material a reader wants when asking why is this the way it is, and exactly the wrong material to meet them in a list of what comes next:

• the DMN acceptance criterion that was unachievable as written ("same decision and receipt" — a receipt pins its pack's identity, so two packs cannot match)
• the equivalence proof that turned out fixture-bounded, and why that motivated pack equivalence in the verifier
• the what-if guard that proves answer-checking alone is insufficient — the kernel confirms the wrong date a broken encoding produces; only the boundary check catches it
• prove's non-zero exit being a differential check between two proof systems, not a routine gate
• the three architecture claims the work sharpened, and the two-halves-of-a-pack distinction
• the corpus never being regenerated once across twenty-one PRs

Older releases get short back-fill entries so the file is complete rather than starting mid-history.

An error caught while condensing

My first draft gave M4 a tenth bullet for what-if queries. What-if is a v1.2 item that shipped early and was already ticked there — M4 legitimately has nine. That is the same mistake that produced the duplicate OR-Tools bullet earlier today: assuming where an item lives instead of checking. Found by counting bullets before and after rather than trusting the edit.

Verification

Docs only. demo/tests 28 passed. Zero orphaned docs, zero broken links or anchors — and a check that every link removed from the roadmap is still reachable from either the README or the changelog, so condensing lost no reference.

CHANGELOG.md is linked from the roadmap intro and from CLAUDE.md's documentation map, so it does not repeat the discoverability failure that #24 and #25 had to clean up.