Implement M6D read-only FAT32 filesystem

[chavalasantosh]

Summary

Implements Soma OS M6D: validated read-only FAT32 persistent filesystem support integrated through the existing block cache and VFS.

Changes

• Adds a read-only FAT32 filesystem implementation.
• Validates the BIOS Parameter Block and FAT32 volume geometry.
• Supports directory traversal and persistent file reads.
• Supports multi-cluster files and offset reads across cluster boundaries.
• Adds long filename handling.
• Detects malformed metadata, invalid clusters, and cluster-chain loops.
• Mounts the FAT32 filesystem through the VFS.
• Preserves the bounded read-only cache policy.
• Keeps persistent storage writes disabled.
• Adds a deterministic FAT32 test-image generator.
• Adds shell and smoke-test coverage for persistent files.
• Updates capability metadata, architecture documentation, threat model, backlog, and validation evidence.
• Preserves all M5, FH1–FH3, and M6A–M6C regression gates.

Validation

• Kernel tests: 69 passed
• Kernel Clippy with warnings denied: passed
• UEFI Clippy with warnings denied: passed
• UEFI release build: passed
• Capability registry check: passed
• Source manifest check: passed
• M6D source checks: passed
• Patch replay against current main: passed
• QEMU smoke test: pending GitHub CI

Safety

The FAT32 implementation is intentionally read-only. Filesystem mutation and persistent disk writes remain disabled until a later milestone introduces transactional write safety and recovery.