Track the remaining structured parser diagnostic migration

## Problem

Wave has a structured `ParseDiagnostic` and unified `WaveError` rendering path, but legacy parser routines still print errors directly to stdout and then return `None`. The top-level parser consequently replaces useful local context with generic messages such as "failed to parse function declaration".

This tracking issue replaces the broad diagnostic requests in #114 and #168 with bounded parser tasks.

## Scope

- Return structured syntax diagnostics from legacy parser routines.
- Preserve the token/span that actually caused the failure.
- Keep human and JSON diagnostic output consistent.
- Remove parser-side error printing as individual grammar paths are migrated.
- Add one positive and one negative regression for each migrated path.

## Out of scope

- Changing Wave language semantics.
- Replacing the existing error crate.
- Adding typo recovery that silently changes the parsed program.

## Completion criteria

- [ ] Declaration and function parser failures preserve their local diagnostic.
- [ ] Control-flow and expression parser failures preserve their local diagnostic.
- [ ] Parser failures do not write ad-hoc error lines to stdout.
- [ ] Human and JSON error formats identify the same source location.
- [ ] Existing valid corpus behavior remains unchanged.

Child issues should each cover one grammar family and remain independently reviewable.

Start by locating the legacy parser routines for declaration, function, control-flow, and expression grammar families, then trace how their failures reach the top-level parser and WaveError rendering. Migrate one grammar family at a time, preserving the failing token/span and removing ad-hoc stdout output. Add one positive and one negative regression per path, and verify human/JSON locations plus the valid corpus remain consistent.

Written by the indexing model from the issue text.