feat(ncp): add L0 storage layer with file backend

Implement content-addressed storage:
- CID to filesystem path mapping (content-addressing)
- Store/retrieve/delete operations
- Directory structure: root/XX/YY/ZZ...
- 100MB default file size limit

Refs RFC-NCP-001 L0 Storage
Developed with Frankie assistance

ncp-prototype/src/l0_storage.nim

@@ -0,0 +1,75 @@
+## l0_storage.nim: L0 Storage Layer for NCP
+## File-based backend with CID content addressing
+## RFC-NCP-001 Implementation
+
+import std/[os, paths, sequtils, hashes]
+import types
+
+## Storage Configuration
+type StorageConfig* = object
+  rootPath*: string           ## Root directory for storage
+  maxFileSize*: int64         ## Max file size (default: 100MB)
+  compression*: bool          ## Enable compression (future)
+
+## L0 Storage Handle
+type L0Storage* = object
+  config*: StorageConfig
+  root*: string
+
+proc initL0Storage*(rootPath: string): L0Storage =
+  ## Initialize L0 Storage with root directory
+  result.config.rootPath = rootPath
+  result.config.maxFileSize = 100 * 1024 * 1024  # 100MB
+  result.root = rootPath
+  
+  # Ensure directory exists
+  createDir(rootPath)
+
+## CID to file path mapping
+## CID: [0x12, 0x34, 0x56, ...] -> path: "root/12/34/5678..."
+proc cidToPath*(storage: L0Storage, cid: CID): string =
+  ## Convert CID to filesystem path (content-addressed)
+  let hex = cid.mapIt(it.toHex(2)).join()
+  result = storage.root / hex[0..1] / hex[2..3] / hex[4..^1]
+
+## Store data and return CID
+proc store*(storage: L0Storage, data: openArray[byte]): CID =
+  ## Store raw data, return CID
+  let cid = generateCID(data)
+  let path = storage.cidToPath(cid)
+  
+  # Create directory structure
+  createDir(parentDir(path))
+  
+  # Write data
+  writeFile(path, data)
+  
+  return cid
+
+## Retrieve data by CID
+proc retrieve*(storage: L0Storage, cid: CID): seq[byte] =
+  ## Retrieve data by CID
+  let path = storage.cidToPath(cid)
+  if fileExists(path):
+    result = readFile(path).toSeq.mapIt(byte(it))
+  else:
+    result = @[]  # Not found
+
+## Check if CID exists
+proc exists*(storage: L0Storage, cid: CID): bool =
+  ## Check if content exists
+  let path = storage.cidToPath(cid)
+  return fileExists(path)
+
+## Delete content by CID
+proc delete*(storage: L0Storage, cid: CID): bool =
+  ## Delete content, return success
+  let path = storage.cidToPath(cid)
+  if fileExists(path):
+    removeFile(path)
+    return true
+  return false
+
+## Export
+export L0Storage, StorageConfig
+export initL0Storage, cidToPath, store, retrieve, exists, delete