Extensible split data sync (#11930)

* Extensible split data sync capability through transactions.

- Split common transport has been split up between the transport layer
  and data layer.
- Split "transactions" model used, with convergence between I2C and
  serial data definitions.
- Slave matrix "generation count" is used to determine if the full slave
  matrix needs to be retrieved.
- Encoders get the same "generation count" treatment.
- All other blocks of data are synchronised when a change is detected.
- All transmissions have a globally-configurable deadline before a
  transmission is forced (`FORCED_SYNC_THROTTLE_MS`, default 100ms).
- Added atomicity for all core-synced data, preventing partial updates
- Added retries to AVR i2c_master's i2c_start, to minimise the number of
  failed transactions when interrupts are disabled on the slave due to
  atomicity checks.
- Some keyboards have had slight modifications made in order to ensure
  that they still build due to firmware size restrictions.

* Fixup LED_MATRIX compile.

* Parameterise ERROR_DISCONNECT_COUNT.

drivers/avr/i2c_slave.h

@@ -22,7 +22,18 @@
 
 #pragma once
 
-#define I2C_SLAVE_REG_COUNT 30
+#ifndef I2C_SLAVE_REG_COUNT
+
+#    if defined(USE_I2C) && defined(SPLIT_COMMON_TRANSACTIONS)
+#        include "transport.h"
+#        define I2C_SLAVE_REG_COUNT sizeof(split_shared_memory_t)
+#    else  // defined(USE_I2C) && defined(SPLIT_COMMON_TRANSACTIONS)
+#        define I2C_SLAVE_REG_COUNT 30
+#    endif  // defined(USE_I2C) && defined(SPLIT_COMMON_TRANSACTIONS)
+
+#endif  // I2C_SLAVE_REG_COUNT
+
+_Static_assert(I2C_SLAVE_REG_COUNT < 256, "I2C target registers must be single byte");
 
 extern volatile uint8_t i2c_slave_reg[I2C_SLAVE_REG_COUNT];