fix: close correctness gaps found by post-refactor adversarial review

Confirmed by a 17-agent review pass over the refactor diff (12 findings
verified against the code and reproduced end-to-end):

- marshalCallArgs: BYREF arguments must match the declared parameter
  type exactly (was: silent bit-reinterpretation, e.g. REAL passed to
  BYREF INTEGER printed garbage with exit 0); whole-array arguments are
  rejected (was: callee assignment smashed the array data pointer ->
  runtime SIGSEGV)
- Null guards for CHR/LCASE/UCASE/IS_NUM/NUM_TO_STR/STR_TO_NUM/ASC
  argument emission (was: compiler SIGSEGV on e.g. CHR(undeclared)),
  matching the guards MID/RIGHT/LEFT already had
- ParseReturnStmt: propagate expression-parse failure (was: diagnostic
  printed but exit 0) and recognize bare RETURN before
  ENDWHILE/UNTIL/NEXT (was: bogus diagnostic on legal early return)
- emitPrototype rejects FUNCTION/PROCEDURE names that collide with
  runtime/libc symbols (was: FUNCTION strlen silently hijacked the
  string runtime; arity mismatch crashed createArgumentAllocas) or with
  builtins (was: silently shadowed); conflicting redefinitions keep the
  first signature
- FOR STEP is evaluated exactly once, before the loop (was: once per
  iteration; the runtime-direction refactor had moved it to N+1)
- RETURN values are coerced to the declared return type via the
  signature table (RETURN n in a RETURNS REAL function now compiles)
- '&' coerces its operands to STRING (CHAR/number operands work);
  OUTPUT of a PROCEDURE call reports a clean error

AGENTS docs updated to match. All positive samples remain byte-identical
to the pre-refactor baseline.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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