Language: throw expressions and Exception object propagation

[PurHur]

Problem

#57 tracks try/catch/finally lowering, but throw and built-in Exception are not usable in app code today. Web apps need throwable errors for validation and HTTP error paths (complements #152 serve 500 mapping).

Goal

throw new Exception('Invalid input');

works in VM; catchable when #57 lands.

Tasks

• Lower PHPCfg\Op\Expr\Throw_ to opcode(s) in lib/Compiler.php
• VM: unwind stack, attach trace (minimal), match Web: Map uncaught exceptions to HTTP 500 in serve driver #152 behavior at top level
• Builtin Exception class (message, code, file, line) — VM object model
• JIT: defer or stub with VM fallback
• PHPT: throw uncaught → exit/non-zero; throw caught → continue
• Integration with bin/serve.php HTTP 500 (Web: Map uncaught exceptions to HTTP 500 in serve driver #152)

Acceptance criteria

Caught exception returns 200 with body; uncaught in serve returns 500 (with PHP_COMPILER_DEBUG).

Dependencies

• Language: try/catch/finally lowering #57 try/catch/finally
• Language: Class methods and property access in JIT #58 methods on objects (for Exception::getMessage)
• Web: Map uncaught exceptions to HTTP 500 in serve driver #152 HTTP error mapping

Files

• lib/Compiler.php, lib/VM.php, lib/OpCode.php, test/compliance/