Parser: `while`, `for`, and `loop` (§5)

[StreamDemon]

Summary

Implement while, for, and loop parsing. All three keywords lex but have no parse production today; each fails with "expected expression".

Part of the v0.6.x parser-completeness work (see crates/AGENTS.md "Implemented subset"). Surfaced during PR #57 verification.

Spec

• §5 control flow; §16 while_expr / for_expr / loop_expr, plus while let (§5.4) and for destructuring (§5.5).
• The block-head restriction applies to the while condition and the for iterable (§5.1/§5.2, §16) — reuse the cond_expr helper from PR Start lexer parser compiler bootstrap #57.

Scope

• while cond { ... } and while let pat = expr { ... }.
• for pat in iter { ... } with destructuring patterns.
• loop { ... }.
• Confirm break / continue (already statements) nest correctly inside these; evaluate break value if §5 allows it.
• AST nodes; corpus fixtures from §5 examples; unit tests incl. the condition/iterable restriction.

Notes

while let and for depend on pattern parsing — coordinate with the match issue so patterns land once and are shared.