Implement Tier 1 Test262 harness requirements for exception handling:
parser (ThrowStmt, TryStmt with boxed data), interpreter (two-level
propagation via StmtResult::Throw and RuntimeError::Thrown), catchable
vs non-catchable error distinction, scope-safe catch parameter binding,
and full finally semantics. Adds expect_property_name so keywords work
after dot (e.g. Promise.catch). Includes 85 new unit tests, 5 JS262
conformance fixtures, and updated documentation.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>