Parse six high-impact real-world Ruby front-end constructs (Round 1)

[tannevaled]

Summary

Round 1 of the parse-conformance push toward 100% on real Ruby apps (Rails / Puppet / stdlib / RuboCop / Chef). Six high-impact front-end constructs, each verified valid against MRI 4.0 (ruby -c) and covered by MRI-parity table tests.

Rails parse-acceptance sweep: 21.12% → 83.09% (723 → 2844 of 3423 .rb files), zero regressions (every file that parsed before still parses).

Constructs landed

1. masgn to non-local targets (the single biggest gap, ~1692 occurrences). The multiple-assignment LHS parser now accepts instance/class/global variables, constants, scoped constants, attribute targets (obj.x), and index targets (arr[i]), with splats on any of them plus nameless * and trailing ,. Non-local targets populate MultiAssign.Targets; attribute/index targets lower to their setter-call shape (x= / []=). The all-locals fast path keeps Targets nil so existing consumers are untouched. A bracket-aware lookahead (scanMlhsTargetTail/scanBalanced) drives the disambiguation.

   • @a, @b, @c = x, y, z · @@a, @@b = … · $a, $b = … · obj.x, obj.y = … · arr[i], h[k] = … · @a, *@b = … · *@a, @b = …

2. rescue/else/ensure without an explicit begin on do…end blocks and class/module/singleton-class bodies (def bodies were already supported), folded into a Begin via the shared parseRescueTail. Brace blocks still reject it, as MRI does.

3. Anonymous + ordered params: bare *, **, & in def and lambda parameter lists (sentinel names */**/&), and kwargs / double-splat followed by a &block param (def f(**options, &block), def f(*args, **opts, &block)). **nil is recorded too.

4. Special global variables in the lexer ($$ $! $@ $/ $\ $; $, $. $< $> $? $* $" $: $+ …), including inside string interpolation ("PID-#{$$}"); previously lexed as ILLEGAL.

5. Value-omitted shorthand keyword arguments (Ruby 3.4) in calls, super, and array literals: foo(format:, name:) ⇒ foo(format: format, name: name). Array literals now share the call-argument grammar, so trailing key: value pairs collapse into one implicit hash and a trailing comma is accepted in both calls and arrays.

6. Single-quoted symbols :'foo.bar' / :'data-remote' (no interpolation, honoring \' and \\), alongside the existing :"…" form.

Quality

• 100% coverage retained (CI gate); go test -race green; gofmt + go vet clean; CGO=0.
• New MRI-parity tests: masgn_targets_test.go, round1_features_test.go, lexer/lexer_round1_test.go.

Downstream (rbgo) follow-ups

These constructs now parse; some need a matching rbgo compiler case to execute (out of scope here — no rbgo changes in this PR):

• Close real-world front-end gaps: ::-paths, and/or/not, masgn consts, ..., %r/%x/%s, keyword method names #1: compiler.storeMultiTarget only handles VarRef/ConstRef. Add cases for IvarRef, CVarRef, GVarRef, ScopedConst, and attribute/index setter Call targets; and guard a nil Targets[i] (nameless * splat).
• Parse six high-impact real-world Ruby front-end constructs (Round 1) #3: anonymous-param sentinels */**/& compile as ordinary local slots; forwarding them as call arguments needs compiler support.
• Add singleton classes, paren-less command args, and panic hardening #2, Parser round 3: 9 grammar features toward 100% parse-conformance (Rails 94.31% → 96.82%) #5, Round 4: close parse-conformance gaps (Rails 96.8% → 99.6%) #6: reuse existing Begin / HashLit / SymbolLit lowering — no new compiler cases expected.

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