feat(backend): add global KV storage fallback for server-side data

[Tjemmmic]

Summary

Adds a server-global key-value storage fallback to MemoryBackend and FileIOBackend for byte-map data that isn't associated with any registered client account.

Previously, byte-map operations (store_byte_map_value, get_byte_map_value, etc.) would silently no-op when session_cid didn't resolve to an existing CNAC — returning Ok(None) / Ok(Default::default()). This made it impossible for servers to persist application-level data (e.g., workspace metadata) using the standard backend KV API, since servers operate without their own CNAC and use CID 0 for global state.

Changes

• memory.rs: Added global_kv: RwLock<HashMap<u64, HashMap<String, HashMap<String, Vec<u8>>>>> field to MemoryBackend. The four byte-map methods (get, remove, store, get_by_key, remove_by_key) now fall back to global_kv when no CNAC matches the session_cid, instead of returning empty results.
• file_io_backend.rs:
  • On connect, loads the global KV store from <server_dir>/global_kv.bin if present.
  • Added save_global_kv() to persist the map via bincode.
  • Mutating byte-map operations now route persistence to save_global_kv() when save_cnac_by_cid returns ClientNonExists, instead of failing the call.
  • get_byte_map_values_by_key no longer attempts to save a CNAC on a read path (drive-by fix).

[codecov]

Codecov Report

❌ Patch coverage is 94.80519% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.6516%. Comparing base (2e9e3fd) to head (21a9271).

Files with missing lines	Patch %	Lines
citadel_user/src/backend/memory.rs	92.7272%	4 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##             master       #243        +/-   ##
================================================
+ Coverage   83.3963%   83.6516%   +0.2553%     
================================================
  Files            91         91                
  Lines         14015      14136       +121     
================================================
+ Hits          11688      11825       +137     
+ Misses         2327       2311        -16     

Files with missing lines	Coverage Δ
citadel_user/src/backend/file_io_backend.rs	96.1783% <100.0000%> (+0.6227%)	⬆️
citadel_user/src/backend/memory.rs	87.7907% <92.7272%> (+10.6822%)	⬆️

... and 8 files with indirect coverage changes

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