refactor(qc): replace moonbitlang/quickcheck with core's QuickCheck Mini

[bobzhang]

Summary

Evaluates and completes the switch from the third-party moonbitlang/quickcheck@0.14.0 test dependency to the QuickCheck framework that now ships in core (moonbitlang/core#3955, "QuickCheck Mini"). Verdict: core covers all of this repo's use cases — the dependency is dropped from moon.mod.

What mapped directly

• Gen → @qc.Generator (bind→flat_map, fmap→map; one_of/frequency/int_range/sized/scale/array_with_size are identical, and int_range is half-open in both)
• @qc.Shrink → @shrink.Shrink — core ships the same String/Int64/Bool/Array instances the shrinkers used
• forall_shrink + quick_check_silence → Arbitrary/Shrink impls for SimpleDocument + @qc.check(count=2000, max_size=12) (deterministic, seeded)

What core lacks, and the workarounds

• char_range/liftA2/liftA3: ~25 lines of local helpers in generator.mbt
• counterexample: the property raises fail(message) carrying the rendered TOML; check reports it plus the Debug-printed shrunk counterexample
• classify coverage stats: replaced by a separate fixed-seed snapshot test tallying the eight contains_* predicates over 2000 samples — distribution matches the old classification output (~33% tables vs 29%, ~67% arrays vs 61%)

Structurally, the generators/shrinkers moved from test-only files into the package proper (generator.mbt, shrink.mbt) so the trait impls satisfy coherence; qc.md is updated.

Caveat

Core's QuickCheck Mini merged 2026-08-03 and is marked WIP upstream — if the stable-channel CI toolchain predates it, the stable job will fail and this should wait for the next stable release.

Test plan

• moon test internal/qc_model — 2/2 pass (2000-case round-trip via @qc.check + coverage snapshot)
• moon test — 412 passed, 0 failed
• moon check / moon fmt / moon info — clean

🤖 Generated with Claude Code

[bobzhang]

CI confirms the caveat: nightly passes, stable fails with Cannot find import 'moonbitlang/core/quickcheck/shrink' — the stable toolchain's core predates moonbitlang/core#3955. The migration is complete and verified on nightly; merge once QuickCheck Mini reaches the stable channel (re-run CI to check).

[bobzhang]

Rebased onto the APIs that actually landed in core main: local char_range/lift2/lift3 helpers are gone in favor of @qc.char_range + zip/zip_with/zip_with3 (core#3977), and the property test + coverage tally merged into a single @qc.report run using the observations API (core#3980) — observe classifies every accepted case and the whole report is snapshotted. With core#3979's Unicode strings, all 2000 round-trip cases still pass under the wider distribution.

Validated locally against core main (2da357d2) via MOON_CORE_OVERRIDE; CI stays red until a toolchain bundles a core snapshot ≥ 2026-08-06 (the current nightly predates the merges).