Fetch: preserve ReadableStream body lifecycle

[chrisbbreuer]

Parent: #406
Related: #405, #141
Roadmap: #134

Make Fetch Body objects preserve engine-native ReadableStream bodies instead of stringifying them at construction. This is the required body lifecycle beneath WebAssembly streaming compilation.

Scope

• Store stream bodies as managed Value edges on Response and Request without eager consumption.
• Expose realm-correct body and derived bodyUsed; distinguish null, buffered, locked, disturbed, closed, errored, and cancelled bodies.
• Route text, JSON, ArrayBuffer, bytes, Blob, and FormData body methods through the same ABI: implement readable-stream consumption and conversions #405 consumer machinery.
• Reject repeated, locked, errored, detached, invalid-chunk, and abrupt-pull consumption through returned Promises.
• Preserve Content-Type inference, cloning rules, sibling realms, GC relocation, cancellation, and failure atomicity.
• Add focused Debug, ReleaseSafe, and TSan tests without running unrelated long corpora.
• Document the exact lifecycle contract and close only after fixing-main CI is green.

[chrisbbreuer]

Implemented on main in 54bbb14c (stream-preserving Fetch Body lifecycle, tee/clone/transfer mechanics, focused runtime coverage) and a35f025e (pinned Home/Bun contract, audit validators, docs, and Debug/ReleaseSafe/TSan CI commands). Local focused Fetch Body and underlying ReadableStream matrices pass in all three modes with zero leaks. Closure remains gated on the exact fixing-main job.

[chrisbbreuer]

Exact fixing-main CI is green, including the complete private-ABI boundary: https://github.com/zig-utils/zig-js/actions/runs/29785272707/job/88495394827