cold: type-erased backend via DynColdStorageBackend

[prestwich]

[Claude Code]

Summary

• Introduces DynColdStorageBackend, an object-safe mirror of ColdStorageBackend with Pin<Box<dyn Future>> returns, auto-implemented for every B: ColdStorageBackend via a blanket impl. Arc<dyn DynColdStorageBackend> is wired to satisfy ColdStorageBackend, and ColdStorage<B> defaults B to that erased form plus a new_erased constructor — so downstream code can write plain ColdStorage without a turbofish when it wants runtime backend swappability, or ColdStorage<MyBackend> for full monomorphization.
• get_logs / produce_log_stream on the erased path eagerly clone the Filter because the unified-lifetime dyn signature cannot bridge the independent-lifetime strong-trait signature without an owned filter. All other methods dispatch with zero overhead. Concrete ColdStorage<B> is unaffected. See the module docs in dyn_backend.rs.
• Adds a conformance_erased entry point so the existing conformance suite runs against Arc<dyn DynColdStorageBackend>, and a focused unit test that verifies a backend's drain_above override survives erasure. Writing those tests caught and fixed an infinite-recursion hazard in the clone-form methods (the cloned Arc<dyn ...> resolved the blanket impl back into the strong-trait impl — fix: qualified-path dispatch via Arc::as_ref()). The recursion hazard is documented in a maintainer note for any future borrowed-arg method.

Test plan

• cargo +nightly fmt -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo clippy --workspace --all-targets --no-default-features -- -D warnings
• `RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps`
• cargo t -p signet-cold --all-features — including new erased_conformance and dyn_drain_above_invokes_backend_override tests

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