Port Bun core ReadableStream stdin matrix

Port the exact upstream test/js/bun/spawn/spawn-stdin-readable-stream.test.ts corpus into Home without whole-file TODO rewriting.

Scope:

• consume string, Uint8Array, pull-driven, delayed, transformed, tee, empty, null-byte, and large streams
• preserve backpressure pull counts and parallel subprocess use
• handle stream errors without unhandled rejections
• reject already-disturbed streams
• cancel stdin exactly once when AbortSignal terminates the subprocess
• preserve immediate success/failure exit codes and resource cleanup
• add a durable Home regression for the exact upstream file

Acceptance:

• corpus remains byte-identical to ~/Code/bun
• 21 active tests pass, 0 fail, 0 unsupported
• 2 explicit upstream TODOs remain
• adjacent stdin/pipe ownership matrices remain green
• full home_test gate passes

Completed in 845b91d.

Evidence:

• Exact upstream corpus is byte-identical to ~/Code/bun/test/js/bun/spawn/spawn-stdin-readable-stream.test.ts.
• Exact matrix: 21 pass, 0 fail, 0 unsupported, 2 explicit upstream TODOs preserved.
• Added promise-backed subprocess stdout, ordered string/byte/pull consumption, partial delivery across stream errors, immediate-exit cancellation, disturbed-stream rejection, AbortSignal cancellation exactly once, backpressure, parallel process, transform, tee, and reader release compatibility.
• Fixed async start() error/close handling so queued bytes drain once without unhandled rejection or pull-loop starvation.
• Adjacent spawn-stdin-pipe-fd-leak.test.ts: 2 pass, 0 fail, 0 unsupported, 1 upstream Zig/Rust divergence TODO.
• Adjacent spawn-unread-stdout-gc.test.ts: 1 pass, 0 fail, 0 unsupported, 0 TODO.
• Full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0.
• No upstream corpus edits; whole-file TODO rewriting removed and a durable 21+2 regression added.