parser: BREAK/CONTINUE tokens reserved in lexer but no production rule uses them #459

Surfaced by

hyperpolymath/standards#284.

Behaviour

`BREAK` and `CONTINUE` are reserved tokens in `lib/parser.mly` but no production rule consumes them. Any use in source produces a syntax error.

```affine
while x < n {
if done { break } // syntax error: BREAK
...
}
```

Why this hurts

Early-exit control flow is idiomatic in TS / JS / Rust. Ports of stream-processing code (line-by-line scans, find-first-match) have to restructure into:

• Combined-guard `while` (`while running && i < n`)
• Sentinel-boolean (`mut done := false; while !done && i < n { ... done := true; ... }`)
• Early `return` from a helper function

All three were used in standards#284 (`s_trim` inner loops, `strip_leading_dot_slash`). Every loop-heavy TS port hits this.

Proposed fix

Add minimal `break` / `continue` to the parser + AST + lowerer:

1. `Ast.expr` gains `Break` / `Continue` constructors (or use the existing exception-style `Throw` infrastructure if that's the chosen lowering).
2. `Parser.mly` adds `BREAK` and `CONTINUE` as expression productions allowed inside `while` / `for` bodies.
3. `Typecheck` checks the construct is lexically inside a loop.
4. Lowering emits the matching JS/Deno construct (or compiles to a flag-and-test if the backend forbids early exits).

Acceptance

• `while \<cond\> { ...; break; ... }` parses + type-checks
• `while \<cond\> { ...; continue; ... }` parses + type-checks
• Loop-context check rejects top-level `break`
• At least one example in `examples/` exercises both
• Test fixture in `test/test_loops.ml` (or wherever loop tests live)

Refs

• feat(scripts): AffineScript port of check-ts-allowlist (TS->AS campaign STEP 2 seed) metadatastician/standards#284 (workarounds documented in PR body "Seam findings")