sdk: add an alloy provider seam over the chain host

[mfw78]

Put an alloy Provider seam over the raw ChainHost::request path so guest strategies call typed provider methods instead of hand-building JSON-RPC, and carry the typed chain method surface through to the guest.

Why

Chain access today is a stringly request(u64, &str, &str) -> String, so authors hand-build JSON-RPC params and parse strings; this is the largest single DX gap from the alloy target. The promised HostTransport: alloy Transport shim is not in the SDK, and the closed chain method RPC enum exists host-side but is never carried to the guest. No other issue covers this. Part of milestone M3: Videre SDK, macros and DX. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Add a HostTransport implementing the alloy Transport over ChainHost::request.
• Expose a guest host.provider(Chain) returning an alloy Provider.
• Carry the typed ChainMethod surface through to the guest.
• Add zero-cost Chain and ChainId newtypes.

Done when

• A guest strategy can call host.provider(chain).get_block_number() and .call(&tx) through an alloy Provider backed by ChainHost.
• The typed ChainMethod surface reaches the guest.
• No hand-rolled JSON-RPC remains at the call sites.

[mfw78]

Landed via #453 (squash-merged into dev/m1 as f902239). Closing manually since the merge target is not the default branch.