Port Bun ReadableStream stdin edge-case matrix

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

Scope:

• pull exceptions with partial data delivery
• early child exit and writes after close
• mixed string, Uint8Array, and ArrayBuffer chunks
• slow consumers, high-frequency chunks, and repeated pulls
• stream reuse prevention and byte streams
• independent stdout/stderr piping
• long single chunks, alternating types, and stdio option variants
• durable exact-file regression

Acceptance:

• corpus remains byte-identical to ~/Code/bun
• all active tests pass with 0 fail and 0 unsupported
• the explicit upstream cancellation TODO remains
• core and integration ReadableStream stdin matrices remain green
• full home_test gate passes

Completed in a828b8f.

Evidence:

• Exact upstream spawn-stdin-readable-stream-edge-cases.test.ts remains byte-identical to ~/Code/bun.
• Exact edge matrix: 12 pass, 0 fail, 0 unsupported, 1 explicit upstream cancellation TODO preserved.
• Pull exceptions preserve every chunk enqueued before the throw, then terminate cleanly.
• Early child exit bounds consumption to the first line and cancels the reader.
• Mixed strings, Uint8Arrays, ArrayBuffers, byte streams, slow consumers, high-frequency chunks, repeated pulls, reuse rejection, split stdout/stderr, 256 KiB chunks, alternating types, and stdio variants are active.
• Core matrix: 21 pass, 0 fail, 0 unsupported, 2 upstream TODOs.
• Integration matrix: 5 pass, 0 fail, 0 unsupported, 0 TODO.
• Full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0.
• Whole-file TODO rewriting was removed and a durable exact-file 12+1 regression was added.