docs: round-trip byte-stability scope + minusOne/minusOneFast state counts

[mellonis]

Summary

Post-merge follow-up to #229. Refines the round-trip stability claim across docs and syncs the in-source state-count comments to match the test fixtures.

What changed

Round-trip stability framing — narrowed

Previous wording across CLAUDE.md, packages/machine/README.md, and test/round-trip.spec.ts said `toMermaid → fromMermaid → toGraph → toMermaid` was "stable for all wrapped states including shared-bare cases" after #174's callable-subtree emit. That overstated the guarantee.

The accurate model:

• Simple wrappers (e.g. `scanToX(eraseHere)`): bytewise stable. The #139 regression test pins this.
• Shared-bare cases (e.g. `library-binary-numbers/minusOne` where `invertNumber` backs both the outermost bare and wrapper-W1's bare): emit a single de-duped node with `&`-joined call arrows; the sharing survives the round-trip. But `GraphNode` ids are runtime `State` ids reassigned on every `fromGraph` rebuild, and a shared bare's post-rebuild id no longer follows the original emission order, so the byte stream reorders. Verify behavioural identity with `equivalentOn → allAgree: true` instead.

Updated in three places:

• `CLAUDE.md` — "Visualization & round-trip" → "Round-trip" paragraph.
• `packages/machine/README.md` — the round-trip callout under the wrapped-state example, AND the v7 alpha.1 changelog entry for #174.
• `test/round-trip.spec.ts` — the explanatory comment above the "bytewise stable for wrapped states" test.

State-count comments synced

`packages/library-binary-numbers/src/index.ts`:

• `minusOne`: `15` → `18` nodes
• `minusOneFast`: `8` → `10` nodes

The `graphs.spec.ts` fixture (lines 17-18) already pins these via `summarizeGraph(graph).stateCount`. The source comments were lagging behind the test-pinned source of truth.

Test plan

• `npm test` — 30 files, 664 tests pass (including `library-binary-numbers state graphs` parametric: 31 cases pass).
• `npm run lint` — clean.
• No runtime behaviour change — doc + comment-only diff.