Port Bun subprocess socketpair shutdown matrix

Port the exact upstream test/js/bun/spawn/spawn-socketpair-shutdown.test.ts corpus into Home without TODO rewriting.

Scope:

• preserve stdout writability across an idle child delay
• cover Python asyncio connect_write_pipe behavior on supported platforms
• preserve stdin readability when the parent writes after an idle delay
• model process/pipe lifecycle and disposal semantics in the native corpus runner
• add a durable Home regression for the exact upstream file

Acceptance:

• exact corpus remains byte-identical to ~/Code/bun
• 3 passed, 0 failed, 0 unsupported, 0 todo on macOS
• adjacent spawn regression matrices remain green
• full home_test gate passes

Implemented and pushed in 4bfc0b7068cf0289590e798376c1e7197e8865f9.

Ported behavior:

• preserves delayed child stdout data across idle pipe periods
• covers Python asyncio connect_write_pipe output on supported platforms
• preserves parent-written stdin through write, flush, and end
• settles exit state and implements kill/ref/unref/resource/disposal lifecycle
• removes the exact upstream corpus file from the TODO rewrite and adds a durable Home regression

Verification:

• exact upstream byte comparison against ~/Code/bun/test/js/bun/spawn/spawn-socketpair-shutdown.test.ts: pass
• spawn-socketpair-shutdown.test.ts: 3 passed, 0 failed, 0 unsupported, 0 todo
• adjacent spawn-signal.test.ts: 4 passed, 0 failed, 0 unsupported, 0 todo
• adjacent spawn-kill-signal.test.ts: 16 passed, 0 failed, 0 unsupported, 0 todo
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: exit 0