Add readable response and FetchEvent vertical

[wieslawsoltes]

Problem

The service-worker control plane can activate and claim a Code OSS webview, but WebScene has no readable response body or FetchEvent.respondWith() vertical. A controlled resource therefore cannot return even a static streamed response, and snapshot-created iframe realms omit the fetch globals entirely.

Change

• add WebIDL-shaped ReadableStream, default reader/controller, cancellation, close/error ordering, locking, and tee()
• make Response own a live body stream with clone/tee and bodyUsed consumption semantics
• add service-worker ExtendableEvent and FetchEvent with client/request attributes, waitUntil(), respondWith(), and handled
• prove controlled synthetic 206 response dispatch and install the same body globals in top-level and nested snapshot realms
• add a WPT-derived Chromium oracle plus native correctness, 100-cycle latency, heap, lifecycle, and queue/RSS regression gates

This is Stack A for #266. The issue stays open. Unchanged Markdown still depends on the next stacked CacheStorage empty-match/request-routing and TransformStream tranche; this PR does not claim full Code broker or product rendering acceptance.

Validation

• service-worker-fetch: pass; 100 cycles, p95 0.208042 ms, V8 heap 1,144,292 → 1,144,292 bytes
• service-worker-lifecycle: pass; 100 cycles, p95 6.03821 ms, registration high-water 1
• service-worker-clients: pass; 100 cycles, p95 0.091792 ms, queue cap 256, peak RSS 49,938,432 → 63,160,320 bytes
• native component profile: 1/1 document, 3/3 subtests, 274 ms
• Chromium 153 oracle: 1/1 document, 3/3 subtests, 110 ms
• V8 bootstrap extraction and Node syntax check: pass
• git diff --check: pass

[wieslawsoltes]

Windows C2026 follow-up is now on the PR head in 823461b3.

• Split the fetch bootstrap into three ordered raw literals: 9,545 / 9,505 / 8,637 bytes (all below the 15,000-byte portability ceiling).
• Concatenated runtime input remains exactly 27,687 bytes with the pre-fix SHA-256 bab3ddbb6bbbeb08907ca91e65f710c5efa3950f6c32102981a529ad1da0962c.
• Added scripts/tests/test_v8_bootstrap_literals.py plus an extractor guard so future oversized bootstrap fragments fail before MSVC compilation.
• Passed Python regression, generated-bootstrap Node syntax check, git diff --check, and the existing native 100-cycle service-worker-fetch gate (p95 0.296 ms; heap 1,144,292 -> 1,144,292).

A local native rebuild could not regenerate because its previously cached V8 source/output tree was removed; the pushed CI run is the clean compilation gate. The unrelated pre-existing IndexedDB C2026 remains outside this PR's ownership.