Fix NVM legacy migration code review issues

[yconst]

Summary

Fixes two code review issues in NVM legacy migration logic with minimal changes:

1. Blocker: Migration now preserves user config by restoring runtime state before saving
2. Medium: Legacy detection now uses full metadata validation instead of single field check

Changes

Fix 1: Migration Preserves Legacy Config

File: firmware/src/nvm/nvm.c (lines 403-426)

Added restore function calls in nvm_wl_migrate_legacy_config() before calling nvm_save_config():

• frames_restore_config()
• ADC_restore_config()
• motor_restore_config()
• sensors_restore_config()
• observers_restore_config()
• controller_restore_config()
• CAN_restore_config()
• traj_planner_restore_config()

This ensures that when nvm_save_config() reads from getters, it gets the legacy config data instead of current runtime state, preserving user settings during migration.

Fix 2: Robust Legacy Detection

File: firmware/src/nvm/nvm.c (lines 373-399)

Replaced single magic_marker check with nvm_wl_validate_metadata() in nvm_wl_detect_legacy_config(). This validates all metadata fields (magic_marker, metadata_version, data_size, and metadata_checksum) instead of just one field, preventing misclassification of corrupted configs.

Implementation Details

Both fixes reuse existing, well-tested functions:

• Fix 1 uses the same restore functions already used in nvm_load_config()
• Fix 2 uses nvm_wl_validate_metadata() already used in scan and load operations

Total lines changed: ~10 lines across 2 functions
New code added: 0 (only calls to existing functions)
Risk: Low (affects only legacy migration path)

Testing

• ✅ Debug build: Success
• ✅ Release build: Success
• ✅ No compilation errors or warnings related to changes
• ⏸️ Hardware tests require device setup (to be run by reviewer)

Edge Cases Considered

1. Corrupted Legacy Config: Migration won't be attempted, device boots with defaults
2. Corrupted New-Format Config: Full validation fails, attempts legacy detection
3. Partial Restore Failure: Restore functions handle invalid data gracefully
4. Migration After CAN ID Change: Legacy CAN ID is preserved correctly