Add pattern chaining support to XBLK format

Pattern entry bytes 108-109 now hold next_pattern (0xFF=loop forever,
0-8=chain to index) and loop_count. MCU will poll LP5562 STATUS register
to detect engine completion and load the successor pattern.

Updated XBLK format spec, Rust Pattern struct (serializer + deserializer),
and React Native app design doc (binary format, data model, resolved
open questions).

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

docs/plans/2026-03-05-eeprom-pattern-format.md

@@ -42,13 +42,27 @@ Offset  Size  Field
 42      32    Engine 2 commands
 74      2     Engine 3 command count
 76      32    Engine 3 commands
-108     4     Reserved
+108     1     Next pattern index (0xFF = loop forever, 0-8 = chain to that index)
+109     1     Loop count (how many full cycles before chaining, 0 = use next_pattern as-is)
+110     2     Reserved
 ```
 
 Max capacity: `(1024 - 16) / 112 = 9` patterns.
 
 The 16-command-per-engine limit is a hardware constraint of the LP5562 (48 bytes engine SRAM). The `branch` command enables infinite looping, so pattern duration is unlimited.
 
+### Pattern Chaining
+
+A pattern can specify a successor via `next_pattern_index`. When set to anything other than 0xFF, the pattern's engine programs should use finite-count `branch(0, N)` instead of `branch(0, 0)` (infinite loop). When all engines stop (detected by polling LP5562 STATUS register 0x0C for engine interrupt bits), the MCU loads the next pattern.
+
+The LP5562 has no interrupt output pin — the MCU must poll STATUS periodically. During the idle loop, a poll every ~500ms is sufficient and costs negligible power vs. the LED current.
+
+Chain examples:
+- `next=0xFF`: Loop forever (default, current behavior)
+- `next=1, loops=0`: Play once, then switch to pattern 1
+- `next=0, loops=3`: Play 3 full cycles, then switch to pattern 0 (can create A→B→A→B sequences)
+- Circular chains (A→B→A) create alternating pattern sequences
+
 ## Boot Sequence
 
 1. Init LP5562 (enable, clock, current)