[Sisyphus] phase-2 / zcc / Wave 3 — Parser (expressions + statements)

## Scope
Parse C11 expressions and statements, completing the parser.

## Deliverables
- Extend `phase2/zcc/src/parse.zig` with expression and statement parsing.
- Extend `phase2/zcc/src/ast.zig` with expression and statement AST nodes.

## Grammar Coverage
- Full expression grammar via precedence climbing / Pratt parser: primary, postfix, unary, cast, multiplicative, additive, shift, relational, equality, bitwise AND/XOR/OR, logical AND/OR, conditional, assignment, comma.
- All statements: labeled (`case`, `default`, identifier-label), compound, expression, selection (`if`, `switch`), iteration (`while`, `do-while`, `for`), jump (`goto`, `continue`, `break`, `return`).
- `_Generic` selection expressions.
- Compound literals.
- Designated initializers (`[index]`, `.member`, range designators).

## Success Criteria
1. Successfully parse reference files `011.c`, `017.c` through `023.c` from `phase1/c11-ref/`.
2. `_Generic` selection in `001.c` and designated initializers in `010.c` parse correctly.
3. AST snapshots are stable across runs.

## Dependencies on Prior Waves
- **Wave 0** — build system.
- **Wave 1** — lexer.
- **Wave 2** — declaration/type parser (expressions depend on type names for casts).

## Suggested Role Assignment
`parser-engineer`

## Test Corpus Expectations
- Parse `011`, `017..023` from `phase1/c11-ref/` and produce AST; compare against golden snapshots.
- Specific tests for `_Generic` and designated initializers.

Start with phase2/zcc/src/parse.zig and phase2/zcc/src/ast.zig, then review the Wave 1 lexer and Wave 2 declaration/type parser dependencies. Run the parser against phase1/c11-ref/011.c and 017.c through 023.c, plus 001.c and 010.c, and compare AST output with golden snapshots; done means all listed grammar cases parse and snapshots are stable.

Written by the indexing model from the issue text.