match arms: accept brace-block bodies

[danieljohnmorris]

Summary

~v:{stmt1;stmt2;final-expr} is now a valid match arm body, alongside the existing inline ~v:stmt1;stmt2;final-expr form. Mirrors the existing =cond{block} grammar so the arm boundary is unambiguous when the body contains call-shapes that resemble patterns.

The manifesto framing: every Result-handling site that wants more than a single expression in an arm currently either uses the inline ; form (which works but is visually noisy and easy for a tired model to mis-segment) or extracts a helper function. Personas reported the helper tax repeatedly (ilo_assessment_feedback.md lines 830, 851, 928). Brace blocks give a visually clear, unambiguous shape that matches what the agent already knows about =cond{block}.

Repro

Before:

go>n;r=num "10";?r{~v:{d=*v 2;+d 1};^e:0}

{"code":"ILO-P009","labels":[...],"message":"expected expression, got LBrace",...}

After:

21

What's in the diff

• parser: accept brace-block bodies in match arms — in parse_arm_body, when the post-colon token is LBrace and it is not a destructure-pattern start, delegate to parse_brace_body. AST shape unchanged: MatchArm.body stays Vec<Spanned<Stmt>>, last stmt is the arm value. Every backend already iterates that way, so no verifier / interpreter / VM / Cranelift change needed.
• tests: cross-engine coverage — tests/regression_match_block.rs with seven tests across tree / VM / Cranelift: single-expr (existing), brace block with local in Ok arm, brace block on Err arm, nested match in block, bool match with blocks on both branches, inline ;-separated still works, match-as-expression with brace-block arm.
• examples: examples/match-block.ilo — engine-asserted example picked up by tests/examples_engines.rs so the shape is exercised on every backend.

Test plan

• cargo fmt
• cargo clippy --release --features cranelift --all-targets -- -D warnings
• cargo test --release --features cranelift — full suite green
• New regression_match_block tests all pass on tree, VM, and Cranelift
• Existing single-expr and inline-; arm bodies still work (anchored by the first and sixth tests)

Follow-ups

None for this PR. The inline-; form remains valid and is the more token-minimal shape for short arm bodies; brace blocks are the readable escape hatch when you need locals or want the boundary to be unmistakable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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