Add pre-dispatch hooks and explicit request ingress fidelity #402

Description

Applications need a local interception seam before method/path routing and ordinary middleware, while preserving the concrete RouterService used by all adapter entry points. Runtime transformations and parser rejection must remain explicit so consumers do not mistake a copied runtime request for original wire preservation.

Add a shared asynchronous pre-dispatch hook and bounded, confidential ingress metadata across Fastly, Cloudflare, Axum, and Spin. Preserve the runtime-visible method token and headers, expose trusted origin facts separately from target fidelity, and document unavailable or unknown capabilities.

Acceptance criteria

• Hooks can return terminal responses, mutate continuing requests, and stop dispatch on errors before router matching/state/introspection/context/middleware.
• All four converters attach ingress metadata; missing or transformed original facts are explicit and never logged or serialized by default.
• Cloudflare conversion preserves Web Request extension-method tokens and UTF-8 header bytes; runtime-level parser rejection remains a distinct result.
• Fastly supports snapshots captured before native mutation without cloning request handles or changing client provenance.
• Locked native/browser/WASM tests and raw local-runtime fixtures validate supported and unsupported outcomes, including origin provenance and process cleanup.
• Required Rust/docs gates pass; capability documentation does not claim production-wire preservation, raw Fastly targets, bounded transport allocation, or consumer compliance.

Verification

Workspace strict format/lint/tests and feature/WASM checks; adapter runtime/browser contracts; 54 source-built fixed raw HTTP cases; isolated fixture/runner format/lint/tests; docs lint/format/build.