sprint-3: PID outer + Heading Hold + ROT feed-forward + gain scheduling

End-to-end implementation per docs/sprint-3-plan.md.

Closes the cascade: outer loop (heading control, 10 Hz on Core 1) drives
the inner loop (rudder position control, 50 Hz from Sprint 2). First real
mode other than STANDBY is now activable: HEADING_HOLD.

Builds: pio run -e esp32-dev SUCCESS, RAM 6.8%, Flash 27.1% (355 KB).
Tests: pytest 258/258 green (231 Sprint 2.5 + 27 Sprint 3 new).

Python (arautopilot/studio/simulator/):

- vessel_heading.py: first-order yaw model. ROT responds to
  rudder*speed; damping returns ROT to zero when rudder is centred.
  Defaults tuned so 5 deg rudder @ 10 kn -> ~3 dps steady-state ROT.
  Includes heading_error_deg() shortest-arc helper.
- pid_outer.py: pure-Python outer heading PID. Anti-windup via back-
  calculation, gain scheduling by SOG, deadband, derivative LPF,
  output saturation, ROT feed-forward (brief sec. 6 -- the term that
  distinguishes a premium autopilot from a basic one), rate limit on
  produced rudder setpoint, shortest-arc heading wrap-around.

Firmware (firmware/ar_autopilot_v1/src/pid/):

- pid_outer.h: header-only C++17 port. Same algorithm, same variables,
  same numerics. Fixed-capacity gain schedule (up to 8 points).
- pid_outer_task.{h,cpp}: 10 Hz FreeRTOS task on Core 1. Subscribes to
  TWDT. Reads heading + ROT from the NMEA 2000 snapshot. Uses
  operator-configurable SOG (default 15 kn until PGN 129026 wiring in
  Sprint 5). Pushes rudder setpoint into the inner loop only when
  current_mode == HEADING_HOLD.

Modes (firmware/ar_autopilot_v1/src/modes/standby.cpp):

- HEADING_HOLD activable via request_mode(). Pre-conditions:
    * NMEA 2000 heading sensor valid (fresh PGN 127250)
    * Rudder sensor valid (median filter filled)
  On success, captures current heading as initial setpoint so the
  operator doesn't get a sudden swing toward an old setpoint.

Modbus (regenerated from YAML):

- 7 new INPUTs (50-56): outer heading setpoint, produced rudder
  setpoint, error, current SOG, live kp/ki/kd.
- 5 new HOLDINGs (24-28): writable heading setpoint, SOG override,
  outer base gains. Writing any of kp/ki/kd disables the built-in
  3-point gain schedule (operator override).

Tests:

- test_vessel_heading_simulator.py: 6 dynamics tests + 9 parameterised
  heading_error_deg edge cases (wrap-around).
- test_pid_outer_python.py: 12 tests covering gain interpolation,
  per-tick PID behaviour (deadband, sign, ROT feed-forward,
  saturation, rate limit, allowed=false), and three end-to-end cascade
  tests (positive step, negative step, wrap-around 360->10).

Cascade verification: outer + inner + rudder dynamics + vessel-heading
simulator settles a 30 deg step within +-2 deg in 60 s.

NOT in Sprint 3 (intentional):
  - True Course / Track Keeping / Dodge -- Sprint 5
  - Off-course alarms + auto-disengage on sensor loss -- Sprint 6
  - COG / SOG / Position via N2K PGN 129025/9/6 -- Sprint 5

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

arautopilot/shared/modbus_register_map.py

@@ -84,6 +84,13 @@ INPUTS: dict[str, Reg] = {
     "PID_INNER_KP_X1000": Reg(addr=43, name="PID_INNER_KP_X1000", desc='Inner-loop kp * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
     "PID_INNER_KI_X1000": Reg(addr=44, name="PID_INNER_KI_X1000", desc='Inner-loop ki * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
     "PID_INNER_KD_X1000": Reg(addr=45, name="PID_INNER_KD_X1000", desc='Inner-loop kd * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
+    "PID_OUTER_HEADING_SP_X100": Reg(addr=50, name="PID_OUTER_HEADING_SP_X100", desc='Outer-loop heading setpoint, deg*100 (0..35999)', unit="deg", scale=0.01, offset=0.0),
+    "PID_OUTER_RUDDER_SP_X100": Reg(addr=51, name="PID_OUTER_RUDDER_SP_X100", desc='Rudder setpoint produced by outer loop, deg*100 (signed int16)', unit="deg", scale=0.01, offset=0.0),
+    "PID_OUTER_ERROR_X100": Reg(addr=52, name="PID_OUTER_ERROR_X100", desc='Outer-loop heading error, deg*100 (signed int16)', unit="deg", scale=0.01, offset=0.0),
+    "PID_OUTER_SPEED_KN_X10": Reg(addr=53, name="PID_OUTER_SPEED_KN_X10", desc='SOG currently used for gain scheduling, knots*10', unit="kn", scale=0.1, offset=0.0),
+    "PID_OUTER_KP_X1000": Reg(addr=54, name="PID_OUTER_KP_X1000", desc='Outer-loop active kp * 1000', unit="", scale=0.001, offset=0.0),
+    "PID_OUTER_KI_X1000": Reg(addr=55, name="PID_OUTER_KI_X1000", desc='Outer-loop active ki * 1000', unit="", scale=0.001, offset=0.0),
+    "PID_OUTER_KD_X1000": Reg(addr=56, name="PID_OUTER_KD_X1000", desc='Outer-loop active kd * 1000', unit="", scale=0.001, offset=0.0),
 }
 
 # HOLDINGS
@@ -97,6 +104,11 @@ HOLDINGS: dict[str, Reg] = {
     "PID_INNER_KP_REQ_X1000": Reg(addr=17, name="PID_INNER_KP_REQ_X1000", desc='Requested inner-loop kp * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
     "PID_INNER_KI_REQ_X1000": Reg(addr=18, name="PID_INNER_KI_REQ_X1000", desc='Requested inner-loop ki * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
     "PID_INNER_KD_REQ_X1000": Reg(addr=19, name="PID_INNER_KD_REQ_X1000", desc='Requested inner-loop kd * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
+    "PID_OUTER_HEADING_SP_REQ_X100": Reg(addr=24, name="PID_OUTER_HEADING_SP_REQ_X100", desc='Requested outer-loop heading setpoint, deg*100 (0..35999)', unit="deg", scale=0.01, offset=0.0),
+    "PID_OUTER_SPEED_KN_REQ_X10": Reg(addr=25, name="PID_OUTER_SPEED_KN_REQ_X10", desc='Requested SOG for gain scheduling, knots*10', unit="kn", scale=0.1, offset=0.0),
+    "PID_OUTER_KP_REQ_X1000": Reg(addr=26, name="PID_OUTER_KP_REQ_X1000", desc='Requested outer-loop base kp * 1000', unit="", scale=0.001, offset=0.0),
+    "PID_OUTER_KI_REQ_X1000": Reg(addr=27, name="PID_OUTER_KI_REQ_X1000", desc='Requested outer-loop base ki * 1000', unit="", scale=0.001, offset=0.0),
+    "PID_OUTER_KD_REQ_X1000": Reg(addr=28, name="PID_OUTER_KD_REQ_X1000", desc='Requested outer-loop base kd * 1000', unit="", scale=0.001, offset=0.0),
 }
 
 ALL_GROUPS: dict[str, dict[str, Reg]] = {