Fix 6 language correctness bugs found in comprehensive audit

[nbeerbower]

CRITICAL fixes:

• Fix labeled break/continue losing target_label through try-finally blocks
  (statements.c: save/restore target_label alongside is_breaking/is_continuing)
• Fix OR patterns in match not binding variables in both interpreter and compiler
  (interpreter: transfer temp_env bindings to parent env on match;
  compiler: pre-declare bindings before OR blocks, use assignment in alternatives)
• Fix compiler defer stack not scoped per function — nested calls executed wrong defers
  (add hml_defer_save_depth/hml_defer_execute_to_depth to runtime,
  emit depth tracking at function entry, use scoped execution at return)

HIGH fixes:

• Fix exception value not retained when re-thrown from task join
  (concurrency.c: VALUE_RETAIN exception_value after copying exception_state)
• Fix const parameter modifier parsed but never enforced at runtime
  (add param_is_const to Function struct, pass to env_define_param,
  environment already enforces const on set)

MODERATE fix:

• Fix leading-dot float literals (.5) not supporting underscore separators
  (lexer.c: allow underscores in fractional/exponent digits, use strip_underscores)

Adds 4 parity tests covering the fixed issues.

https://claude.ai/code/session_01Vm74RMmHxnBri5cPBSshkY