feat: Gleam-style multi-subject ? match

[danieljohnmorris]

Summary

• Adds multi-subject ? match: ?a b{"ok" "ok":"both"; _ _:"miss"} — each arm has N space-separated patterns, one per subject
• Stmt::Match/Expr::Match field subject: Option<Expr> renamed to subjects: Vec<Expr>; single-subject and subject-less forms are unchanged
• New Pattern::Tuple(Vec<Pattern>) variant for multi-subject arms; interpreter, register VM, verifier, and codegen (fmt + python) all updated
• Parser lookahead (looks_like_multi_subject + first_arm_has_n_patterns) prevents false-positive disambiguation against existing ternary hint shapes (ILO-P009 regression suite still green)

Test plan

• cargo test — all 5000+ tests pass (including regression_ternary_brace_hint 6/6)
• ?a b{"ok" "ok":"both";_ _:"miss"} returns "both" via --vm
• ?a b{"ok" "ok":"both";_ _:"miss"} with non-matching subjects returns "miss"
• Engine-matrix file 50-match-multi-subject.ilo added

Closes

ILO-412

🤖 Generated with Claude Code

[danieljohnmorris]

needs manual rebase (conflicts in: src/ast/mod.rs src/codegen/fmt.rs src/codegen/python.rs src/interpreter/mod.rs src/verify.rs src/vm/mod.rs)

[codecov]

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