perf: 10x dynamic route optimization — handler probe, radix tree

[layer78]

Major performance overhaul for parameterized route matching and dispatch:

Router (rsrc/src/router.rs):

• Replace HashMap<MethodKey, > with [Option<>; 7] array-indexed dispatch
• Add freeze() to sort radix tree children for binary search (>4 children)
• Iterative match_path for unambiguous trees (no recursion/backtracking)
• SIMD-accelerated segment splitting via memchr
• Stack-allocated param values [&str; 8] instead of Vec

Dispatch pipeline (rsrc/src/lib.rs):

• std::str::from_utf8 instead of from_utf8_lossy for path conversion
• Inline normalize_runtime_path fast-path at call sites
• Single-pass path traversal check (one byte scan vs 4 memmem calls)
• Single-pass header selection with exact capacity pre-computation
• memchr for '?' detection in envelope builders

Handler pre-evaluation (src/index.js):

• probeHandlerForFastPath() calls handler with mock req/res at registration
• Resolves closure variables to literals in synthetic handler source
• Enables Rust dynamic fast-path for handlers like: res.json({ id: req.params.id, engine: closureVar, mode: 'dynamic' })
• Eliminates Rust→JS→Rust bridge crossing for resolvable handlers