Add firmware verification harness

Requirements.md

@@ -47,7 +47,7 @@ Sender state machine invariants must remain behavior-equivalent:
   - frame format: `[msg_kind][short_id_be][payload][crc16_ccitt]`.
   - reject invalid CRC/msg-kind/length.
 - Payload codec:
-  - schema `3` with `present_mask` (30-bit sparse second map).
+  - schema `4` with `present_mask` (30-bit sparse second map).
   - support `n==0` sync-request packets.
 - Time bootstrap guardrail:
   - sender must not run normal sampling/transmit until valid ACK time received.
@@ -221,7 +221,7 @@ Sender state machine invariants must remain behavior-equivalent:
 ## `lib/dd3_legacy_core/src/payload_codec.cpp`
 
 - `bool encode_batch(const BatchInput&, uint8_t*, size_t, size_t*)`
-  - schema v3 encoder with metadata, sparse present mask, delta coding.
+  - schema v4 encoder with metadata, sparse present mask, delta coding.
 - `bool decode_batch(const uint8_t*, size_t, BatchInput*)`
   - strict schema/magic/flags decode + bounds checks.
 - Varint primitives:
@@ -475,7 +475,7 @@ Current core orchestration requirements:
 ## 6. Rust Porting Constraints and Recommendations
 
 - Preserve wire compatibility first:
-  - LoRa frame byte layout, CRC16, ACK format, payload schema v3.
+  - LoRa frame byte layout, CRC16, ACK format, payload schema v4.
   - sender optimization changes must not alter payload field meanings.
 - Preserve persistent storage keys:
   - Preferences keys (`ssid`, `pass`, `mqhost`, `mqport`, `mquser`, `mqpass`, `ntp1`, `ntp2`, `webuser`, `webpass`, `valid`).