Integrate turboapi-core: shared radix trie router + HTTP utilities

[justrach]

Context

turboAPI and merjs now share a common Zig library: turboapi-core (justrach/turboapi-core). It provides:

• Radix trie router — method-aware, {param} + *wildcard support, fuzz-tested, O(log n) matching
• HTTP utilities — queryStringGet, percentDecode, statusText, formatHttpDate
• Bounded response cache — thread-safe BoundedCache(V) with max entry cap

The dependency is already wired in build.zig.zon and build.zig (via mer_mod.addImport("turboapi-core", core_mod)). All merjs tests pass.

What's done

• turboapi-core library created with router, http utils, cache
• turboAPI-python wired to import router from core (router.zig deleted from turboAPI)
• merjs build.zig.zon + build.zig wired (path dep for now)

What's next

• API router upgrade: Use turboapi-core.Router for api/ routes to get method-based dispatch (GET vs POST on same path)
• Replace queryParamFromStr in request.zig with core.http.queryStringGet (identical signature, needs module import plumbing)
• Optional: Replace linear-scan dynamic route matching with radix trie for page routes (perf win for large route counts)
• Move to git URL dep once justrach/turboapi-core repo is published on GitHub