Code review improvements from CodeRabbit:
1. ShardBits Guardrails (ec_volume_info.go):
- AddShardId, RemoveShardId: Reject shard IDs >= MaxShardCount
- HasShardId: Return false for out-of-range shard IDs
- Prevents silent no-ops from bit shifts with invalid IDs
2. Future-Proof Regex (disk_location_ec.go):
- Updated regex from \.ec[0-9][0-9] to \.ec\d{2,3}
- Now matches .ec00 through .ec999 (currently .ec00-.ec31 used)
- Supports future increases to MaxShardCount beyond 99
3. Better Error Messages (volume_grpc_erasure_coding.go):
- Include valid range (1..32) in dataShards validation error
- Helps operators quickly identify the problem
4. Validation Before Save (volume_grpc_erasure_coding.go):
- Validate ECContext (DataShards > 0, ParityShards > 0, Total <= MaxShardCount)
- Log EC config being saved to .vif for debugging
- Prevents writing invalid configs to disk
These changes improve robustness and debuggability without changing
core functionality.