Migrate match arm grammar errors to structured diagnostics

Tracked by #346.

## Why

The match parser still prints errors for malformed `=>`, case qualification, payload separators, arm bodies, and duplicate wildcards. These failures should use the compiler's structured diagnostic path.

## Scope

- Convert the syntax-level failure paths in `front/parser/src/parser/control.rs` for match headers, patterns, arrows, and arm bodies.
- Preserve semantic variant exhaustiveness diagnostics as they are.
- Add negative tests for a missing `=>`, missing case after `::`, malformed payload list, missing arm brace, and duplicate wildcard.
- Do not change match or variant semantics.

## Completion criteria

- [ ] No direct print remains in the match grammar paths covered here.
- [ ] Errors identify the match/pattern context and expected token.
- [ ] Tests assert message and source location.
- [ ] Existing variant frontend tests pass.

Start in front/parser/src/parser/control.rs and trace the match grammar failure paths for headers, patterns, arrows, and arm bodies. Add negative tests for the five listed malformed cases, asserting diagnostic messages and source locations. Done means those paths use structured diagnostics, semantic exhaustiveness diagnostics remain unchanged, and existing variant frontend tests pass.

Written by the indexing model from the issue text.