fix(storage): keep !Send MDBX write tx out of async state machine

[prestwich]

Summary

• Extract synchronous hot-storage unwind logic from drain_above into a new unwind_hot_above helper, keeping the !Send MDBX write transaction out of the async state machine
• Add compile-time Send canaries for all async methods on UnifiedStorage<DatabaseEnv> (drain_above, cold_lag, replay_to_cold) and StorageBuilder::build
• Bump workspace version to 0.6.7

Context

UnifiedStorage::drain_above() held an MDBX Tx<Rw, PtrUnsync> (which is !Send) across an .await point, making the returned future !Send. This broke reth::install_exex which requires Send futures.

All other writer()/reader() call sites were audited — the remaining uses are in synchronous code (conformance tests, test utilities, write_hot, unwind_above) and not at risk. cold_lag holds a !Send reader but it's dead before the .await, so the compiler correctly excludes it from the state machine. The canary guards against regressions there.

Closes ENG-2080

Test plan

• cargo clippy -p signet-storage --all-features --all-targets
• cargo clippy -p signet-storage --no-default-features --all-targets
• cargo t -p signet-storage — all 9 tests pass
• Compile canary fails without the fix, passes with it
• Audited all writer()/reader() uses across the repo for !Send across .await

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