Port Bun S3 cancelled-download stream lifecycle

Tracks js/bun/s3/s3-stream-cancel-leak.test.ts under #66 and #81.

Scope:

• expose S3File.stream() through the Home Bun bootstrap
• return a pull-driven byte stream that supports reader cancellation
• release all source state on cancel without retaining the stream wrapper
• support the fixture’s direct bun:jsc heapStats import
• activate the exact copied GC regression and add a permanent Home gate

Close once the exact fixture and full Home gate pass.

Implemented and published in 6a93bfbe4.

Evidence:

• exact copied js/bun/s3/s3-stream-cancel-leak.test.ts: 1 passed, 0 failed, 0 TODO, 0 unsupported
• S3File.stream() is now pull-driven and cancellable; cancellation clears the source state without retaining the stream wrapper
• bun:jsc heapStats import is activated through the Home bootstrap module bridge
• permanent corpus regression added
• full ./pantry/.bin/zig build test -Dfilter=home_test --summary failures gate passed

The deterministic cancellation/GC lifecycle fixture is fully active, so this focused issue can close.