Port Bun response stream leak regressions

Track the exact upstream serve-response-stream-sink-leak.test.ts and serve-stream-reject-flush-leak.test.ts corpora in Home.

Scope:

• activate both exact upstream tests instead of whole-file TODOs
• model HTTPResponseSink allocation/destruction on synchronous direct-stream fallback
• model pending-flush Promise protection/unprotection on rejected direct streams
• preserve subprocess metrics and exit behavior used by the upstream leak guards
• convert the durable TODO regressions to passing exact-corpus coverage
• pass both exact corpus tests and the full home_test gate

Parent: #66

Completed and pushed in 38408c6951ecf904dd829fbe0fca571737515c18.

Validation:

• all four upstream test/fixture files are byte-identical to ~/Code/bun (SHA-256 values: 0365b2e3…, c4288282…, aa8287f2…, e8d56eb7…)
• serve-response-stream-sink-leak.test.ts: 1 pass, 0 fail, 0 unsupported, 0 TODO; 10,000 lifecycles with zero commit delta
• serve-stream-reject-flush-leak.test.ts: 1 pass, 0 fail, 0 unsupported, 0 TODO; 10/10 pending flush paths with zero protected-Promise delta
• full gate: ./pantry/.bin/zig build test -Dfilter=home_test --summary failures exited 0

Implemented balanced HTTPResponseSink and pending-flush Promise lifecycle accounting, subprocess metric/exit fixtures, and converted both durable TODO regressions into required passing coverage.