refactor(host): make HostState generic over the component backends

[mfw78]

What

Makes HostState generic over the component seam: HostState<C, W, S, H> (chain, cow, store handle, http), with all ten WIT Host impls rewritten as generic impls bounded on the seam traits, a Components bundle threading the shared backends through a generic Supervisor and event loop, and the concrete pools wired in bootstrap. Store and linker construction are deduplicated behind build_store and build_linker, removing the near-identical blocks the module-restart path carried.

Three typing improvements ride the rewrite, since these are exactly the signatures being rewritten:

• Chain ids are typed as alloy_chains::Chain end to end: the seam traits, both pools, the event loop, the supervisor, and the engine.toml chains map keys (HashMap<Chain, ChainConfig>; numeric TOML keys stay canonical and named keys such as [chains.sepolia] now also parse; no existing config file changes). Every metric label, structured log field, and the persisted last_dispatched_block key keeps the numeric id, pinned by a new test, so observable identifiers and stored data are unchanged.
• The CoW passthrough method is typed as http::Method, with the four-method allowlist and its error text preserved and the guest string uppercased before parsing at the WIT glue.
• HttpClient::fetch speaks http::Request<Vec<u8>> / http::Response<Vec<u8>> with an explicit timeout, keeping the seam free of bindgen types; conversion lives in the glue and the stub's guest-visible message is byte-identical.

Why

The concrete pools were baked into HostState, so backends could not be swapped or faked and every host impl was welded to production types. This is the structural step that lets the following change collapse the runtime's shape into a single associated-type lattice parameter, and it was the one moment the chain-id, method, and HTTP typing could land without touching the same ten impl blocks twice.

Closes #87. Closes #101. Closes #102. Closes #104.

Testing

Executed in five ordered slices, each passing the full workspace gate (fmt, clippy with -D warnings, rustdoc with -D warnings, tests) before the next began. New tests: named-key and invalid-key config parsing, and the progress-marker key format pin for a named chain. Final audits confirm no BTreeMap<u64, _> remains, no Chain reaches a metric label, log field, or persisted key through its Display form, and the WIT boundary still speaks plain u64.

AI Assistance: Claude Code used for implementation, adversarial review, and PR preparation.