Fix memory leaks & blocking syscall in Bun Shell (#23636)

## Summary

Fixes two critical bugs in Bun Shell:

1. **Memory leaks & incorrect GC reporting**: Shell objects weren't
reporting their memory usage to JavaScriptCore's garbage collector,
causing memory to accumulate unchecked. Also fixes a leak where
`ShellArgs` wasn't being freed in `Interpreter.finalize()`.

2. **Blocking I/O on macOS**: Fixes a bug where writing large amounts of
data (>1MB) to pipes would block the main thread on macOS. The issue:
`sendto()` with `MSG_NOWAIT` flag blocks on macOS despite the flag, so
we now avoid the socket fast path unless the socket is already
non-blocking.

## Changes

- Adds `memoryCost()` and `estimatedSize()` implementations across shell
AST nodes, interpreter, and I/O structures
- Reports estimated memory size to JavaScriptCore GC via
`vm.heap.reportExtraMemoryAllocated()`
- Fixes missing `this.args.deinit()` call in interpreter finalization
- Fixes `BabyList.memoryCost()` to return bytes, not element count
- Conditionally uses socket fast path in IOWriter based on platform and
socket state

## Test plan

- [x] New test: `shell-leak-args.test.ts` - validates memory doesn't
leak during parsing/execution
- [x] New test: `shell-blocking-pipe.test.ts` - validates large pipe
writes don't block the main thread
- [x] Existing shell tests pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Claude Bot <claude-bot@bun.sh>

src/bun.js/api/ParsedShellScript.classes.ts

@@ -0,0 +1,30 @@
+import { define } from "../../codegen/class-definitions";
+
+export default [
+  define({
+    name: "ParsedShellScript",
+    construct: true,
+    noConstructor: true,
+    finalize: true,
+    hasPendingActivity: false,
+    configurable: false,
+    valuesArray: true,
+    memoryCost: true,
+    estimatedSize: true,
+    klass: {},
+    proto: {
+      setCwd: {
+        fn: "setCwd",
+        length: 1,
+      },
+      setEnv: {
+        fn: "setEnv",
+        length: 1,
+      },
+      setQuiet: {
+        fn: "setQuiet",
+        length: 1,
+      },
+    },
+  }),
+];