ci: add moon check --deny-warn gate

[bobzhang]

Missing gate: ci.yml ran moon check / moon -C tests check without --deny-warn (fmt gate already present via moon fmt && git diff --exit-code).

Change: ci.yml -> moon check --deny-warn and moon -C tests check --deny-warn.

Warning fixes (196 warnings -> 0, on every target)

1. Generator pins bumped (parser/moon.pkg: moonbitlang/yacc@0.7.18 -> 0.7.23, moonbitlang/ulex@0.3.29 -> 0.3.30; codegen/codeblock_parser/moon.pkg: ulex@0.3.29 -> 0.3.30). The newer yacc emits explicit extend declarations and no inexhaustive guards, which removes ~60 warnings from parser/parser.mbt. parser.mbt regenerated; lexer.mbt/codeblock_parser.mbt output is unchanged by the ulex bump.
2. implicit_impl_as_method (89): explicit pub extend T with Trait::{...} next to each type/impl for exactly the methods the warning lists, so the public API is unchanged (pkg.generated.mbti now lists these methods; the diff is additions only). Token's ToJson (from the %derive in parser.mbty, which yacc doesn't extend) is extended in the hand-written parser/parser_show.mbt.
3. guard_inexhaustive (34): guard ... else { panic() }, which spells out the panic the guard already did. That includes the two guards in the parser.mbty grammar actions.
4. deprecated: StringBuilder::new() -> StringBuilder() (in codegen, parser/lexer.mbtx and, in the tests module, jsonlexer.mbtx, bytes.mbtx, lexbuf.mbt; generated lexers regenerated). The Show impls of Token/ParseError call Debug::to_repr(self) instead of the deprecated to_repr method, with the same output.
5. unused_package: removed moonbitlang/ulex/regex (root), core/int (codegen, only used inside a string template), util/eof_char_set (dot_tests), util/bounded_enum (util/char_set), and parser's black-box-test-only core/test (no black-box tests there; the wbtest import is kept). core/debug in util/diet moved to a for "test" import.
6. parser/__snapshot__/spec.json updated: it snapshots the parsed lexer.mbtx, whose embedded code now reads StringBuilder(). Nothing else in it changed.

Local verification (moon 0.1.20260920): moon info --target all / moon fmt leave no diff (root and tests), moon check --deny-warn clean (root, also --target all) and moon -C tests check --deny-warn clean, moon test 38/38 and moon -C tests test 11/11 (same as before).

Decisions for review

• Generator pin bump (yacc 0.7.23 / ulex 0.3.30) instead of hand-editing generated output.
• All extends are plain pub (including Debug::{to_repr}) to keep the API identical. If some of these promotions aren't meant to be public (e.g. to_repr, or the util/automaton internals), you can mark those extends #deprecated, or remove them.
• pub extend Token with ToJson::{to_json} sits in hand-written parser_show.mbt, because the yacc %derive doesn't generate it.
• else { panic() } is used for the inexhaustive guards to preserve behaviour. Some of these may deserve real error handling.

Update for moon 0.1.20261012 / moonc v0.10.15 (added to this PR) — CI stays red until two releases

On the new toolchain neither master nor this branch can be built: the dependency moonbitlang/ulex-runtime@0.4.3 (the latest published version) ships a moon.mod.json, which is now a hard error ("is no longer supported. Run moon fmt to migrate"). moonbitlang/moonlex-runtime is already converted on its default branch, but not released yet.

Blocked on:

1. a new release of moonbitlang/ulex-runtime (with moon.mod). Then bump the pin in moon.mod and tests/moon.mod.
2. a new release of moonbitlang/yacc whose runtime template no longer emits @list.empty() (now deprecated in favour of List([])). With yacc@0.7.23 (the latest) the regenerated parser/parser.mbt has one such call (line 1674, state_stack), which fails moon check --deny-warn. Then bump the pin in parser/moon.pkg.

What this commit already does, so that only the two pin bumps remain:

• moon fmt in the root module and in tests/ (Int → Int32, UInt → UInt32, x? : T = v → x~ : T = v): formatter output.
• Deprecations named by the compiler: @list.empty() → @list.List([]) in the two grammar actions of parser/parser.mbty (parser.mbt regenerated by the build rule), x.to_json() → Json(x) in util/char_set, subset → is_subset in automaton.

Verified locally by replacing the unpacked ulex-runtime@0.4.3 in .mooncakes with the converted sources from moonlex-runtime's default branch (same version, same code; nothing of that is committed): moon info --target all and moon fmt leave no diff in both modules, moon -C tests check --deny-warn is clean, moon test 38/38 and moon -C tests test 11/11 (same as before). moon check --deny-warn in the root module reports exactly one remaining error, the generated line from item 2.

Also seen, not changed: moon prints "exclude in moon.mod is deprecated; use .gitignore or .moonignore" for options(exclude: [ "tests" ]) in both moon.mod files, and "moon runwasm is deprecated ... use moonx instead" for the build rules. Both are notices from moon, they do not fail the gates.

Note: source using Int32/UInt32 needs moon 0.1.20261012 or newer.