Enable wasm GC support (native)

[konsumer]

Summary

• WAMR_BUILD_GC 1 in host/cmake/Findwamr.cmake - native host now accepts modules using wasm-GC struct types
• No code changes needed elsewhere: cart_gc()/cart_alloc_tracked manage a separate linear-memory arena for host-returned struct/string buffers, unrelated to WAMR's own GC heap, which the runtime collects automatically
• Web host needs no change - it already runs carts through the browser's own WebAssembly engine (not WAMR), and modern browsers ship wasm-GC already
• README: new "probably will not support" section documenting Kotlin/dart2wasm/janet with concrete reasons
• Doc accuracy pass across .claude/skills/ and AGENTS.md (see below)

Verification

• Hand-written .wat cart using struct.new/struct.get + a null0 import loads and runs cleanly under the GC-enabled host for 300+ frames, zero errors
• Full clean rebuild (rm -rf build), confirmed -DWASM_ENABLE_GC=1 actually compiled into 52 build steps and GC symbols (wasm_obj_is_struct_obj etc.) present in the binary - not a stale/cached build
• Regression pass: all 18 supported cart languages (24 example carts) rebuilt fresh via their docker images and run against the GC-enabled host - all ran clean for their full duration, no crashes or new errors

What's still blocked (documented in the skill, not fixed here)

• Kotlin/Wasm's wasm-wasi target is genuinely close (standalone WASI + custom-module imports, real GC struct usage) but hits a real WAMR loader gap: it unconditionally rejects any GC array type with anyref elements, which sits deep in Kotlin's stdlib. Also needs -Xwasm-use-traps-instead-of-exceptions since WAMR's fast interpreter and exception-handling are a hard-incompatible build combination.
• dart2wasm's default output isn't standalone - it imports a dart2wasm JS namespace and the JS-string-builtins proposal, ruled out by the existing "no JS glue" rule, same as emscripten.

Doc accuracy pass

Went through AGENTS.md and every .claude/skills/*/SKILL.md checking claims against the actual codebase:

• AGENTS.md: fixed the stale "No wasm GC proposal" line, added detail on the fast-interp/exception-handling build incompatibility
• add-api-function/SKILL.md: added Tilemap/i32[] (existing types the skill's list omitted) and gui.yml/tile.yml (existing API groups missing from the example list)
• tools/docker/README.md: python was listed under "experimental (can't call null0 host-functions yet)" and missing from the build/publish command lists - confirmed by screenshot that python carts do call host functions correctly (clear/draw_rectangle render fine), moved it into the normal list
• Regenerated API.md and a few language bindings that had a stale NULL0_VERSION constant (npm run gen hadn't been re-run since the last version bump)
• fix-host-bug/SKILL.md, make-cart/SKILL.md + references/languages.md checked against source - no drift found

Test plan

• CI passes (no host behavior change for existing carts)