docs: complete Phase 6.5.6 - Advanced CID API documentation

  Comprehensive documentation for the Advanced CID API:

  - docs/API.md: Add 500+ line Advanced CID API section with complete examples
    - FS5Advanced class (pathToCID, cidToPath, getByCID, putByCID, putWithCID, getMetadataWithCID)
    - CID utilities (formatCID, parseCID, verifyCID, cidToString)
    - 10+ code examples showing real-world usage
    - When to use guidance (advanced vs path-based API)
    - Bundle size and type definitions reference

  - README.md: Add Advanced CID API section with quick start
    - Import examples for s5/advanced export
    - Complete workflow example
    - Available methods list
    - Updated bundle sizes (Advanced: 59.53 KB)

  - docs/IMPLEMENTATION.md: Mark Phase 6.5 complete
    - All 6 sub-phases complete (6.5.1 through 6.5.6)
    - 74/74 tests passing (38 CID utils + 36 FS5Advanced)
    - Achievement: Clean architectural separation maintained

  Phase 6.5 now 100% complete with comprehensive documentation.

Author: Developer

README.md

@@ -255,24 +255,91 @@ For production deployments, these benchmarks confirm the implementation is ready
 The library supports multiple import strategies to optimize bundle size:
 
 ```javascript
-// Full bundle (~273KB uncompressed, ~70KB gzipped)
-import { S5, MediaProcessor } from "@s5-dev/s5js";
+// Full bundle (~60KB compressed with brotli)
+import { S5, MediaProcessor } from "s5";
 
-// Core only - no media features (~195KB uncompressed, ~51KB gzipped)
+// Core only - no media features (~60KB compressed)
 import { S5, FS5 } from "s5/core";
 
-// Media only - for lazy loading (~79KB uncompressed, ~19KB gzipped)
+// Media only - for lazy loading (~10KB compressed)
 import { MediaProcessor } from "s5/media";
 
+// Advanced CID API - for power users (~60KB compressed)
+import { FS5Advanced, formatCID, parseCID } from "s5/advanced";
+
 // Dynamic import for code-splitting
 const { MediaProcessor } = await import("s5/media");
 ```
 
 Monitor bundle sizes with:
 ```bash
-node scripts/analyze-bundle.js
+npm run analyze-bundle
+```
+
+## Advanced CID API
+
+For power users who need direct access to Content Identifiers (CIDs), the Advanced API provides content-addressed storage capabilities without affecting the simplicity of the path-based API.
+
+### When to Use
+
+**Use the Advanced API if you:**
+- Need to reference content by its cryptographic hash
+- Are building content-addressed storage applications
+- Require deduplication or content verification
+- Work with distributed systems that use CIDs
+
+**Use the Path-based API if you:**
+- Need simple file storage (most use cases)
+- Prefer traditional file system operations
+- Want paths to be more meaningful than hashes
+
+### Quick Example
+
+```typescript
+import { S5 } from "s5";
+import { FS5Advanced, formatCID, parseCID } from "s5/advanced";
+
+// Setup
+const s5 = await S5.create();
+await s5.recoverIdentityFromSeedPhrase(seedPhrase);
+const advanced = new FS5Advanced(s5.fs);
+
+// Store data and get both path and CID
+const result = await advanced.putWithCID('home/document.txt', 'Important data');
+console.log(`Path: ${result.path}`);
+console.log(`CID: ${formatCID(result.cid, 'base32')}`);
+
+// Share the CID string
+const cidString = formatCID(result.cid, 'base58btc');
+
+// Recipient: retrieve by CID alone
+const receivedCID = parseCID(cidString);
+const data = await advanced.getByCID(receivedCID);
+console.log(data); // "Important data"
+
+// Find path from CID
+const path = await advanced.cidToPath(receivedCID);
+console.log(path); // "home/document.txt"
 ```
 
+### Available Methods
+
+**FS5Advanced Class:**
+- `pathToCID(path)` - Extract CID from file/directory path
+- `cidToPath(cid)` - Find path for a given CID
+- `getByCID(cid)` - Retrieve data by CID
+- `putByCID(data)` - Store data and return CID
+- `putWithCID(path, data)` - Store and get both path and CID
+- `getMetadataWithCID(path)` - Get metadata with CID
+
+**CID Utilities:**
+- `formatCID(cid, encoding?)` - Format CID as multibase string
+- `parseCID(cidString)` - Parse CID from string
+- `verifyCID(cid, data, crypto)` - Verify CID matches data
+- `cidToString(cid)` - Convert to hex string
+
+See the [Advanced API Documentation](./docs/API.md#advanced-cid-api) for complete details.
+
 ## Encryption
 
 Enhanced S5.js includes **built-in encryption** using XChaCha20-Poly1305, providing both confidentiality and integrity for sensitive data.