Port Bun ReadableStream stdin integration matrix

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

Scope:

• documentation-style ReadableStream stdin echo
• HTTP Response body piped into subprocess stdin
• TransformStream output passed to subprocess
• 100-chunk large stream ordering
• realtime CSV processing with line aggregation
• durable exact-file regression and adjacent core stream coverage

Acceptance:

• corpus remains byte-identical to ~/Code/bun
• all 5 active tests pass with 0 fail, 0 unsupported, 0 TODO
• core ReadableStream stdin matrix remains green
• full home_test gate passes

Completed in eb04f11.

Evidence:

• Exact upstream spawn-stdin-readable-stream-integration.test.ts remains byte-identical to ~/Code/bun.
• Exact integration matrix: 5 pass, 0 fail, 0 unsupported, 0 TODO.
• HTTP Response.body is consumed through its async-iterable stream reader and marks the response body used.
• Documentation echo, TransformStream output, 100-chunk ordering, readline line counting, and realtime CSV averaging are active.
• Adjacent core spawn-stdin-readable-stream.test.ts: 21 pass, 0 fail, 0 unsupported, 2 explicit upstream TODOs.
• Full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0.
• Whole-file TODO rewriting was removed and a durable exact-file 5/0/0/0 regression was added.