Add SRAM mailbox protocol with all 7 command handlers

Implements the NFC-to-MCU SRAM mailbox protocol in src/ntag5/sram.rs:
- Command parsing with CRC-16/CCITT-FALSE verification
- Response building with 4-byte-aligned SRAM writes
- WRITE_PATTERN: stores 112-byte pattern entries to EEPROM
- GET_STATUS: returns firmware version + library summary
- SET_ACTIVE: changes the active pattern index in EEPROM header
- SYNC_START/SYNC_END: bulk upload with deferred header CRC
- READ_LIBRARY/READ_NEXT: iterative pattern readback
- MailboxState struct for cross-command state tracking

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ntag5/mod.rs

@@ -9,6 +9,8 @@
 /// - READ REGISTER: write [BL_AD1, BL_AD0, REGA], read 1 byte
 /// - WRITE REGISTER: write [BL_AD1, BL_AD0, REGA, MASK, REGDATA]
 
+pub mod sram;
+
 use embedded_hal::i2c::I2c;
 
 pub const DEFAULT_ADDRESS: u8 = 0x54;