Migrate inline assembly parser failures to structured diagnostics

Tracked by #346.

## Why

`front/parser/src/parser/asm.rs` and the asm-expression parser still report malformed blocks and operands with direct prints. Inline assembly is error-prone, so consistent source-located diagnostics are especially important.

## Scope

- Convert parser failures for missing braces/parentheses, malformed `in`/`out`, malformed `clobber`, and invalid operand syntax.
- Keep backend constraint validation and target-specific assembly behavior out of scope.
- Add compact negative parser tests for each converted family.
- Preserve the current inline assembly grammar.

## Completion criteria

- [ ] Converted paths return structured errors without printing.
- [ ] Diagnostics name the asm construct being parsed.
- [ ] Tests cover statement and expression asm forms.
- [ ] Valid existing inline-asm cases remain unchanged.

Start in front/parser/src/parser/asm.rs and trace the asm-expression parser paths that directly print failures. Convert the listed malformed-brace, operand, in/out, clobber, and invalid-syntax cases to source-located structured errors while leaving grammar and backend validation unchanged. Add compact negative coverage for statement and expression forms, then verify valid inline-asm cases remain unchanged.

Written by the indexing model from the issue text.