[codegen/closures] preserve concrete class info on captured class instances

[cs01]

Before

A closure that captured a class instance by value lost the concrete class name. Inside the lambda body, capturedVar.field couldn't find the class struct for GEP, so the load used the field's nominal type against whatever alloca held the capture. Pointer/string fields segfaulted; number fields returned garbage.

class T { h: string = "hello"; }
const t = new T();
setTimeout(() => { console.log(t.h); }, 10); // printed garbage

After

Captured class instances retain their concrete class in the inner scope, so capturedVar.field generates the correct GEP + load. The repro above prints hello.

Description

Closes #646. Step 3b of #640.

Threads the concreteClass of outer-scope vars from the symbol table through ClosureAnalyzer and into CapturedVariable records, then — inside the lambda body — calls symbolTable.setConcreteClass(name, className) on the re-bound capture after the normal defineVariable call.

Why this isn't PR #644 (closed)

PR #644 took the same plumbing path but re-bound captures via defineVariableWithMetadata(..., createClassMetadata(...)) instead of defineVariable. That completely swapped the symbol's metadata object — losing fields like isPointerAlloca that other codegen paths depended on — and stage 1 self-hosting silently stopped rejecting "hello" - 5 in the binary-type-in-function fixture.

This PR keeps the existing defineVariable call untouched and only adds the concreteClass field to the already-defined symbol via the existing setConcreteClass setter. No metadata is replaced. The binary-type-in-function fixture still fails to compile as expected on stages 0/1/2.

~82 LOC across 10 files.

Verification

• Full npm run verify (stages 0/1/2) green.
• tests/fixtures/semantic/binary-type-in-function.ts still emits the cannot use '-' between 'string' and 'number' compile error.
• New fixture tests/fixtures/closures-cabi/closure-captures-class-field-string.ts prints TEST_PASSED on stages 0/1/2.

[github-actions]

Benchmark Results (Linux x86-64)

Benchmark	C	ChadScript	Go	Node	Place
Fibonacci	0.816s	0.765s	1.562s	3.168s	🥇
Hash Map Lookup	0.091s	0.062s	0.090s	0.118s	🥇
Binary Trees	1.358s	1.241s	2.827s	1.200s	🥈
Cold Start	0.8ms	0.9ms	1.2ms	27.0ms	🥈
File I/O	0.118s	0.091s	0.086s	0.200s	🥈
JSON Parse/Stringify	0.037s	0.051s	0.175s	0.133s	🥈
N-Body Simulation	1.670s	2.118s	2.210s	2.394s	🥈
Regex Match	0.016s	0.005s	0.021s	0.004s	🥈
SQLite	0.052s	0.369s	0.490s	0.409s	🥈
Monte Carlo Pi	0.389s	0.412s	0.405s	2.251s	🥉
Quicksort	0.214s	0.247s	0.213s	0.262s	🥉
Sieve of Eratosthenes	0.014s	0.027s	0.018s	0.039s	🥉
String Manipulation	0.008s	0.019s	0.017s	0.037s	🥉
Matrix Multiply	0.439s	0.715s	0.609s	0.377s	#4

CLI Tool Benchmarks

Benchmark	ChadScript	grep	node	xxd	Place
Hex Dump	0.645s	—	0.881s	0.130s	🥈
Recursive Grep	0.022s	0.011s	0.106s	—	🥈