Migrate type alias, enum, and variant declaration errors to structured diagnostics

Tracked by #346.

## Why

Declaration parsing in `front/parser/src/parser/decl.rs` still prints many syntax errors for `type`, integer `enum`, and payload `variant` declarations before returning failure.

## Scope

- Convert syntax failures for these three declaration families to structured parser errors.
- Cover missing names, separators, representation types, payload types, commas, semicolons, and closing braces.
- Keep semantic checks such as cycles, duplicate cases, and variant exhaustiveness unchanged.
- Do not merge the distinct enum and variant models.

## Completion criteria

- [ ] Converted declaration failures no longer print directly.
- [ ] Each declaration family has positive and negative regression coverage.
- [ ] Diagnostics retain context, expected/found information, and a useful location.
- [ ] Existing generic and variant frontend tests pass.

Start in front/parser/src/parser/decl.rs and trace parsing for type aliases, integer enums, and payload variants. Use the existing generic and variant frontend tests as the regression-test entry points, covering positive and negative cases. Done means declaration syntax failures are structured, retain context, expected/found information, and location, while semantic checks and existing tests remain unchanged.

Written by the indexing model from the issue text.