fix(stream/web): track writer backpressure desired size

[andrewtdiz]

Summary

• count in-flight and queued WritableStream writes when computing writer.desiredSize
• replace and resolve writer.ready as backpressure appears/drains
• read dynamic WritableStream strategy highWaterMark values during lowering
• remove two now-green stream/web known failures

Verification

• cargo fmt --check
• jq empty test-parity/known_failures.json
• cargo check -p perry -p perry-codegen -p perry-stdlib
• ./run_parity_tests.sh --suite node-suite --module stream/web --filter desired-size (3 pass; parity_report_20260529_205526.json)
• ./run_parity_tests.sh --suite node-suite --module stream/web --filter writer-ready (2 pass; parity_report_20260529_205621.json)

Notes

• node-suite/stream/web/writer-write-resolves-after-sink still fails separately because Perry resolves before the async sink body awaits; left in known_failures.