wasmtime host: from_material — Rust-held keys as typed handles (#395)

[lannbot]

Closes #395.

The native parity for the #392 seams: per-kind constructors on the wasmtime host that place embedder-held signature key material in the store's resource table as typed handles — SigningKey::from_material(view, SigningKeyMaterial) and VerifyingKey::from_material(view, SigPublic). No WIT change, no guest-visible change: host-implementation API, like the polyengine seam. Unblocks polyvisor's engine-host filling its app-owned device-identity import (bindgen! with-maps the signature resources onto these types; the returned Resource<_> is what its import hands the guest).

Mechanics

• Same table, same accounting as a mint: the constructor charges the retention pool (floor-only for these kinds) and the reservation travels in the resource, releasing when it leaves the table. An exhausted budget reports the same recoverable other condition a guest mint does.
• Kind validated at the wrap by construction: unlike a CryptoKey, SigningKeyMaterial/SigPublic carry every binding losslessly — algorithm, ECDSA curve/digest, PSS salt, policy — so the polyengine seam's family-refusal matrix has nothing to refuse here. Every family the core can represent injects; the existing getters answer from the supplied material and policy.
• SigningKeyMaterial, SigPublic, SigningPolicy, and the core Error are re-exported so embedders build material without a version-locked direct dependency on the core crate.

Co-dependent core change (the seam makes the gap reachable): import_ed25519_seed and import_ecdsa_scalar were the only public material constructors skipping the at-least-one-usage mint check — both also cited WIT functions that no longer exist. They now enforce check_useful like every sibling, so injection cannot admit a usage-less key the WIT mints refuse (the polyengine seam's not-permitted refusal, relocated to material construction). No guest-visible effect: every WIT path already checked before reaching them.

Scope: signing/verifying now (the blocked consumer); derivation kinds on demand, mirroring the browser seam's v1.

Gates: just fmt-check, just clippy, just test (new core refusal tests, embedder_key_tests — getter/sign-verify round trip through injected handles, retention-pool parity with delete-readmits — and the runnable doctest), just rust-docs warning-free.