whuppi/ci 2.0.0

Two gate changes. MAJOR: a consumer must update its Makefile to adopt.

• Added tool/verify_web_gate.sh, the shared dual-compiler web gate. It compiles
  a consumer's example under both flutter build web (dart2js) and --wasm
  (dart2wasm). The two compilers have different type models, so js-interop code
  dart2js accepts (a non-exhaustive JSAny switch, an unsound interop cast)
  dart2wasm can reject, and nothing else in the toolchain compiles wasm (the
  analyzer and dart2js use the JS model; pana's wasm tag is an import heuristic).
  A JS-only build is a false green. Registered in stamped-files.txt.
• The stamped gates no longer default their SDK/config env vars. A
  ${VAR:-fvm dart} fallback silently diverges a laptop from CI, so
  analyze_core, platforms_gate, and verify_web_gate now require
  DART / FLUTTER / EXPECTED_PLATFORMS and fail loud if unset. The one
  default lives in the consumer's Makefile, which passes them explicitly.

Adopting: re-stamp all gates, pass EXPECTED_PLATFORMS from make platforms,
and wire make verify-web to the new gate.

Commits since v1.0.8 (3)

• 8b11c52 release: v2.0.0
• 3815270 docs: 2.0.0 changelog — gates require inputs + web gate (#21)
• 712cb91 feat: shared dual-compiler web gate (dart2js + dart2wasm) (#20)