Add the core VM execution engine for the JavaScript bytecode compiler:

- Value type with all JS primitives (Number, String, Boolean, Null,
Undefined, Object, Function)
- Register file with call frame base offsets for nested calls
- Instruction dispatch loop executing all ~50 bytecode opcodes
- Arithmetic with type coercion (ToNumber), string concatenation
- Comparison with abstract/strict equality (ECMA-262 §7.2.14/15)
- Bitwise operations with ToInt32/ToUint32 conversion
- Control flow: jumps, conditional branches, loops
- Function calls: push/pop call frames, argument passing, returns
- Native function support (fn(&[Value]) -> Result<Value, RuntimeError>)
- Object property get/set (plain objects with prototype chain)
- Exception handling with throw and handler stack unwinding
- typeof, instanceof, in, void, delete operators
- Global variable storage via HashMap
- Stack overflow protection (512 frame limit)

Also store function declarations as globals so recursive calls work,
update evaluate() to return completion values, and fix test262 harness
for new signature.

40+ unit tests and end-to-end tests including fibonacci, factorial,
loops, objects, string concat, and type coercion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Fix five correctness bugs in the bytecode compiler:

1. Switch default case corrupted bytecode — patch_jump(0) overwrote the
first instruction when a default case was present. Rewrote switch
compilation to use separate tracking for case jumps vs default fallthrough.

2. For-loop continue jumped to wrong target — continue inside a for-loop
body jumped before the body instead of to the update expression. Changed
continue to use forward-jump patching (like break) so the target is
resolved after body compilation.

3. Do-while continue jumped to wrong target — continue jumped to the body
start instead of the condition check. Same forward-jump patching fix.

4. Empty constructor had no Return instruction — classes without explicit
constructors created a Function with empty bytecode. Now emits
LoadUndefined + Return.

5. Class expression didn't compile methods — only the constructor was
compiled for class expressions. Now compiles methods as properties,
matching class declaration behavior.

Also adds patch_jump_to(pos, target) to BytecodeBuilder for patching
jumps to arbitrary targets (not just current offset).

6 new regression tests. All 182 JS tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add register-based bytecode instruction set designed for JIT compilation
and a recursive-descent compiler that transforms the parser AST into
bytecode functions.

New files:
- bytecode.rs — Instruction set (50+ opcodes), constant pool, function
objects, bytecode builder with jump patching, and disassembler
- compiler.rs — AST → bytecode compiler with greedy register allocation,
scope-aware local variable binding, and support for all statement and
expression types

Instruction categories: register loads, arithmetic, bitwise, comparison,
logical, control flow (jumps, calls, returns), object/property access,
closures, and small-integer optimization (LoadInt8).

Compiler handles: variable declarations, function/class/arrow declarations,
if/else, for/while/do-while loops with break/continue/labels, switch,
try/catch, member access, assignment (simple and compound), logical
short-circuit, conditional expressions, template literals, destructuring,
object/array literals, and nested function compilation.

52 unit tests covering bytecode encoding, constant pool dedup, jump
patching, disassembler output, and compilation of all major grammar
productions. All 176 workspace JS tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>