sprint-8: EKF + adaptive tuner + HWID + SHA-256 audit hash-chain

- heading_ekf.py: 2-state Kalman filter fusing PGN 127250 heading and
  127251 ROT with shortest-arc innovation and symmetric covariance update
- adaptive_tuner.py: gradient-descent outer-loop Kp/Ki adjuster bounded
  to ±adaptive_max_deviation_pct; oscillation vs steady-state detection
- hwid.py: HMAC-SHA256 activation token (verify side); hwid_from_mac_words
  converts three Modbus uint16 MAC words to 12-char hex HWID
- audit.py: SHA-256 hash-chain -- each JSONL line carries prev_hash and
  line_hash; verify_chain() detects tampering, deletion, insertion
- firmware/system/hwid.h+cpp: esp_efuse_mac_get_default wrapper + FNV-32
  hash + "AA:BB:CC:DD:EE:FF" formatter
- modbus_registers.yaml + generated .h/.py: HWID_MAC_01/23/45 at
  input addrs 9/10/11 (three 16-bit words = 6-byte MAC)
- modbus_slave.cpp: INPUT_HWID_MAC_01/23/45 cases read eFuse MAC
- main.cpp: logs HWID string + FNV-32 hash at boot (activation traceability)
- tests: 72 new tests (audit signing, EKF, adaptive tuner, HWID) -- 398 total

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

firmware/ar_autopilot_v1/src/protocols/modbus_slave.cpp

@@ -35,6 +35,7 @@
 #include "nmea2000_consumer.h"
 #include "../hal/knob_encoder.h"
 #include "../safety/safety_monitor.h"
+#include "../system/hwid.h"
 
 namespace arautopilot::protocols::modbus {
 
@@ -97,6 +98,17 @@ uint16_t read_input_register(uint16_t addr) {
         case INPUT_FREE_HEAP_KB:     return (uint16_t)(ESP.getFreeHeap() / 1024U);
         case INPUT_MIN_FREE_HEAP_KB: return (uint16_t)(ESP.getMinFreeHeap() / 1024U);
 
+        // Sprint 8: Hardware ID (MAC eFuse, 3 × uint16)
+        case INPUT_HWID_MAC_01:
+        case INPUT_HWID_MAC_23:
+        case INPUT_HWID_MAC_45: {
+            uint8_t mac[6] = {};
+            system::hwid_get_mac(mac);
+            if (addr == INPUT_HWID_MAC_01) return (uint16_t)((mac[0] << 8) | mac[1]);
+            if (addr == INPUT_HWID_MAC_23) return (uint16_t)((mac[2] << 8) | mac[3]);
+            return                                (uint16_t)((mac[4] << 8) | mac[5]);
+        }
+
         case INPUT_RUDDER_ANGLE_DEG_X100: {
             auto r = hal::rudder_sensor_latest();
             int v = (int)(r.angle_deg * 100.0f);