Fetch API backed by net crate #284

Implements issue 3mhyfoli6gc2g — Fetch API backed by net crate.

Changes #

• New file: crates/js/src/fetch.rs — Fetch API implementation

  • fetch(url, options?) global function returning Promise<Response>
  • Background thread for non-blocking network I/O via we-net HTTP client
  • Response object with status, statusText, ok, url, headers, text(), json()
  • Headers object with get(), has(), set(), delete() (case-insensitive)
  • Thread-local pending fetch state with GC root tracking
  • Event loop integration: completed fetches resolve/reject promises between ticks

• Modified: crates/js/src/vm.rs

  • run_event_loop and pump_event_loop now drain completed fetches alongside timers
  • collect_roots includes pending fetch promise GC roots
  • New drain_completed_fetches() method

• Modified: crates/js/src/builtins.rs

  • Exposed json_parse_pub() for Response.json() to reuse JSON parsing
  • Added init_promise_proto_for_test() for standalone test setup

• Modified: crates/js/Cargo.toml — Added we-net and we-url dependencies

Tests #

7 new tests covering:

• fetch is a function global
• fetch() returns a promise object
• fetch() with no args throws TypeError
• Network error (connection refused) rejects the promise
• Response object has correct properties (status, ok, statusText, url)
• Headers case-insensitive storage and lookup
• Duplicate header combining with ", "