Fix #191: reachability sweep drops return-only-generic orphan records

[jagguji]

Summary

A function whose type parameter appears only in its return (jsonBoxed<T>(): BoxOf<T>) has that return flagged — a return-only 'a can't round-trip (contract rule #4). But classify had already registered the record it built for the discarded return, and nothing un-registered it, so an unreferenced boxOf<'a> reached the output — and shoved the real readBox(): BoxOf<string> type off the clean base name to boxOfV1hnll.

The per-site snapshot/rollback used for the method path (#189) does not work here: the orphan survives through late name/key resolution at emit (#128 + stabilizeNames run after the rollback). So this adds the post-traversal reachability sweep that #178 explicitly called for.

What it does

sweepUnreachableEntries — after every IR tree is final but before stabilizeNames:

• walks the emitted roots (component props + baseSpreads, function/const/context signatures, class members) and drops any registered entry unreachable from them;
• is key-based (every module-mode ref is keyed via refTo), so a type reached only through a keyed ref whose home is late-bound (Callable-module home placement ignores prop-derived deps (deferred from #103 review) #128) is never wrongly dropped;
• uses a deep walk (collectAllRefKeys) so refs inside inline-record fields and record spreads — which the shallow collectRefKeys skips — are covered;
• runs before naming, so a live sibling reclaims a base name an orphan squatted;
• removing an orphan can also dissolve a spurious SCC merge it was wedged into → cleaner module homes.

Before / after

jsonBoxed<T>(): BoxOf<T> + readBox(): BoxOf<string>:

// before
type boxOfV5tl5y<'a> = { v: 'a }   // orphan — referenced by nothing
type boxOfV1hnll   = { v: string }
// after
type boxOf = { v: string }         // clean; jsonBoxed still flagged unit => string

Validation

• npm test — 116 golden cases match; zero existing goldens changed (no orphans there), only the new fixture added.
• npm run test:compile — all 116 compile on ReScript.
• npm run bench — real orphan removals surfaced and accepted: @smastrom/react-rating (inputProps<'a>, a callback-return generic) and @juspay/blend-design-system (plus a spurious HighchartsSharedTypes SCC merge dissolving into ChartsTypes); all metrics equal-or-better, all compile. Baselines updated in this PR.
• Fixture: return-only-generic-orphan; docs/TYPE_MAPPING.md + CHANGELOG updated.

The #189 method-path rollback is kept as a cheap early exit; the sweep is now the general backstop (and the mechanism #178 asked for).

Fixes #191

🤖 Generated with Claude Code

[github-actions]

Benchmark: ✅ PASS

Package	Compile	Diff vs baseline	usable	review	broken	Verdict
@juspay/blend-design-system@0.0.36	✅	identical	102	5	0	✅ PASS
@juspay/blend-design-system@0.0.37-beta.8	✅	identical	215	7	0	✅ PASS
@juspay/blend-design-system@0.0.37	✅	identical	219	7	0	✅ PASS
react-day-picker@10.0.1	✅	identical	19	7	0	✅ PASS
react-tooltip@6.0.7	✅	identical	1	0	0	✅ PASS
react-markdown@10.1.0	✅	identical	0	2	0	✅ PASS
@smastrom/react-rating@1.5.0	✅	identical	1	0	0	✅ PASS
clsx@2.1.1	✅	identical	0	0	0	✅ PASS
hono@4.12.25	✅	identical	0	0	0	✅ PASS
@base-ui-components/react@1.0.0-rc.0	✅	identical	174	21	0	✅ PASS

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commit: 99d68cc