Migrate postfix and aggregate expression errors to structured diagnostics

Tracked by #346.

## Why

Method calls, indexing, qualified calls, grouped expressions, arrays, and struct literals still have ad-hoc parser prints in `front/parser/src/expr/`.

## Scope

- Convert missing member names, closing delimiters, struct field separators, and qualified-name failures to structured parser errors.
- Cover postfix calls/indexing and primary aggregate expressions.
- Add focused negative tests that assert the construct context and source location.
- Do not change expression precedence, method dispatch, or aggregate semantics.

## Completion criteria

- [ ] Converted expression paths return errors without direct output.
- [ ] Group, call, index, array, struct literal, and qualified-name failures are covered.
- [ ] Valid nested postfix expressions still parse.
- [ ] Human and JSON error modes remain consistent.

Begin in front/parser/src/expr/ and trace the postfix, primary aggregate, grouped-expression, call, index, array, struct-literal, and qualified-name parsing paths that still print errors directly. Add focused negative tests for construct context and source location, then verify valid nested postfix expressions and consistency between human and JSON error modes without changing precedence, dispatch, or aggregate semantics.

Written by the indexing model from the issue text.