feat(storage): auto-detect layout changes and re-initialize storage

[Raymond8196]

Summary
When users change layers, rows, cols, or encoder count in keyboard.toml and reflash, the old stored keymap in flash persists and causes incorrect behavior (e.g., Vial showing wrong layer count). Currently the only workaround is manually setting clear_storage = true for one flash cycle, which is error-prone and hard to discover.
This PR replaces the unused BUILD_HASH mechanism with a layout structure hash that automatically detects incompatible changes and re-initializes storage.

Changes

• rmk/src/config/mod.rs: Add layout_hash: u32 field to StorageConfig (defaults to 0)
• rmk-macro/src/codegen/chip/flash.rs: Compute FNV-1a hash of (rows, cols, layers, num_encoder) at compile time and pass it through StorageConfig
• rmk/src/storage/mod.rs:
  • Rename LocalStorageConfig.build_hash to layout_hash
  • Enable the previously commented-out hash comparison in check_enable()
  • Add diagnostic log when layout hash mismatch is detected
  • Remove unused BUILD_HASH import

Design Decisions

• Only structural dimensions are hashed (rows, cols, layers, num_encoder) — not keymap content. Changing a key binding within the same dimensions won't wipe Vial customizations. Users who want to force new defaults can still use clear_storage = true or clear_layout = true.
• Backward compatible: use_rust examples use ..Default::default() which sets layout_hash: 0. On first boot after upgrade, the old build_hash value won't match, triggering a one-time re-initialization — correct migration behavior.
• Why not fix BUILD_HASH? It included chrono::Local::now(), changing every build. A content-based hash of layout dimensions is the right granularity.

[github-actions]

Binary Size Report

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[HaoboGu]

This doesn't solve the issue. It's rare that the col/row/num_layer/num_encoder changes, what's more common is to update the keymap/config. Currently if the layout changes, the keyboard will auto re-initialize the storage and reboot.