Implement local solar time face

Full implementation of the local solar time complication using the
pveducation.org formula set (LSTM, B, EoT, TC, LST, HRA). EoT and TC
are cached by day-of-year and recomputed once at midnight rollover.

Three display modes cycle with the Alarm button:
  SO HH:MM:SS  — Local Solar Time
  nO HH:MM     — Solar Noon in local clock time
  Hr ±DDD      — Hour Angle in degrees

Location is read from location.u32 on the filesystem. In the simulator
the browser lat/lon globals are written to location.u32 on activation
if not already set, fixing the "no Loc" issue in the emulator.

watch-faces/complication/local_solar_time_face.h

@@ -0,0 +1,84 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2025 Raffael Mancini
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#pragma once
+
+/*
+ * LOCAL SOLAR TIME FACE
+ *
+ * Displays solar time information based on the user's location.
+ * Formulas follow the notation from:
+ *   https://www.pveducation.org/pvcdrom/properties-of-sunlight/solar-time
+ *
+ * Variables (pveducation.org notation):
+ *   LSTM  - Local Standard Time Meridian [degrees]  = 15 * ΔTUTC
+ *   B     - Seasonal angle [degrees]                = (360/365) * (d - 81)
+ *   EoT   - Equation of Time [minutes]              = 9.87*sin(2B) - 7.53*cos(B) - 1.5*sin(B)
+ *   TC    - Time Correction Factor [minutes]        = 4*(Longitude - LSTM) + EoT
+ *   LST   - Local Solar Time [hours]                = LT + TC/60
+ *   HRA   - Hour Angle [degrees]                    = 15*(LST - 12)
+ *
+ * B, EoT and TC only depend on the day-of-year d, so they are cached
+ * in state and recomputed exactly once per day: the cache key is d itself
+ * (1-366). Zero-initialisation of state guarantees a recompute on first use.
+ *
+ * Requires the location to be set via the Sunrise/Sunset face, stored in
+ * "location.u32" on the filesystem.  If no location is set, displays
+ * "SO  no Loc".
+ *
+ * Display modes (cycle with the Alarm / start-stop button):
+ *   SO  HH:MM:SS  — Local Solar Time (LST), live seconds display
+ *   nO  HH:MM     — Solar Noon in local clock time
+ *   Hr  ±DDD      — Hour Angle (HRA) in degrees; negative=morning, positive=afternoon
+ */
+
+#include "movement.h"
+
+typedef enum {
+    LOCAL_SOLAR_TIME_MODE_LST  = 0,  /* Local Solar Time    SO HH:MM:SS */
+    LOCAL_SOLAR_TIME_MODE_NOON = 1,  /* Solar Noon (local)  nO HH:MM    */
+    LOCAL_SOLAR_TIME_MODE_HRA  = 2,  /* Hour Angle          Hr ±DDD     */
+    LOCAL_SOLAR_TIME_NUM_MODES
+} local_solar_time_mode_t;
+
+typedef struct {
+    local_solar_time_mode_t mode;
+    uint16_t last_calc_d;  /* day-of-year (1-366) when EoT/TC were last computed;
+                              0 (zero-init) guarantees recomputation on first tick */
+    float    EoT;          /* Equation of Time [minutes]       */
+    float    TC;           /* Time Correction Factor [minutes] */
+} local_solar_time_state_t;
+
+void local_solar_time_face_setup(uint8_t watch_face_index, void **context_ptr);
+void local_solar_time_face_activate(void *context);
+bool local_solar_time_face_loop(movement_event_t event, void *context);
+void local_solar_time_face_resign(void *context);
+
+#define local_solar_time_face ((const watch_face_t){ \
+    local_solar_time_face_setup,    \
+    local_solar_time_face_activate, \
+    local_solar_time_face_loop,     \
+    local_solar_time_face_resign,   \
+    NULL,                           \
+})