refactor(cold): require &mut self for ColdStorage write methods

[prestwich]

Summary

Split ColdStorage into three traits for compile-time read/write separation:

• ColdStorageRead: Clone + Send + Sync, all &self read methods — cloneable, shareable across tasks
• ColdStorageWrite: Send, &mut self write methods (append_block, append_blocks, truncate_above) — exclusively owned
• ColdStorage: supertrait combining both, provides drain_above with default implementation

Eliminate RwLock from the task runner. The runner now holds:

• An owned write backend (exclusive, for inline writes)
• A cloned read backend (shared via Arc for spawned read tasks)

Read tasks literally cannot call write methods at the type level.

Also adds drain_above override to EitherCold to dispatch to inner backend's atomic implementation (was using the non-atomic default).

Closes ENG-1979

Test plan

• cargo clippy clean on all 4 affected crates with both --all-features and --no-default-features
• mem_backend_conformance passes
• mdbx_backend_conformance passes
• All unified storage integration tests pass (including drain_above_*)
• ./scripts/test-postgres.sh (requires Postgres)

🤖 Generated with Claude Code

[prestwich]

[Claude Code]

Reworked based on your review feedback:

1. Docs removed — docs/superpowers/ files dropped from the commit.
2. EitherCold drain_above — now overrides to dispatch to inner backend's atomic impl.
3. Trait split — ColdStorage split into ColdStorageRead (Clone + Send + Sync, &self) + ColdStorageWrite (Send, &mut self) + ColdStorage supertrait (provides drain_above). RwLock eliminated from the task runner — read tasks get a cloned read backend via Arc, writes use an exclusively owned backend. Read tasks cannot call write methods at the type level.

Ready for another look.