feat(gps): add timer-based assisted reacquisition

Remove the fixed pseudorange start offset, preserve per-PRN navigation assistance across RF loss and power cycles, and seed coherent navigation measurement phase on reacquisition. Add full LNAV almanac visibility scheduling, power-safe QSPI journaling, RF-domain reset isolation, and PPS Gray CDC hardening with Linux, sanitizer, RTL, and end-to-end coverage.

soc/software/pps_python/pps_model.py

@@ -5,7 +5,8 @@ mirrors the conventions used by gps_system/src/gps_ip.c and NavEquations.c:
 
 * corrected_abs = raw_abs - rem_code_phase / CODE_FREQ_BASIS
 * modulo-32 ABS_SAMPLE values are unwrapped around the newest channel
-* obs_i = c * ((abs_i - min(abs)) / SAMPLES_PER_CODE + STARTOFFSET) / 1000
+* obs_i/c = (corrected_abs_i - corrected_abs_ref) / RF_HZ
+            - (transmit_i - transmit_ref)
 * satellite clock correction is added to the observation
 * [x, y, z, common_bias_m] are solved together with URA/elevation weights
 
@@ -27,8 +28,6 @@ RF_HZ = 16_367_981.0
 TRACKING_HZ = 125_000_000.0
 SAMPLES_PER_CODE = 16_367.981  # physical RF samples/ms used by the C code
 CODE_FREQ_BASIS = 1.023e6
-START_OFFSET_MS = 68.802
-START_OFFSET_S = START_OFFSET_MS / 1000.0
 NAV_SOLVE_PERIOD_S = 10 * 0.020
 POSTPONE_TOW_S = 0.180
 OMEGA_EARTH = 7.2921151467e-5
@@ -680,10 +679,14 @@ class ReceiverEpochSimulator:
         corrected_abs = np.asarray(
             [measurement.corrected_abs_unwrapped for measurement in measurements]
         )
-        ps_tvl_ms = corrected_abs / SAMPLES_PER_CODE
-        reference_index = int(np.argmin(ps_tvl_ms))
-        precise_tx_ms = ps_tvl_ms - ps_tvl_ms[reference_index] + START_OFFSET_MS
-        observations_m = precise_tx_ms * (C / 1000.0)
+        reference_index = int(np.argmin(corrected_abs))
+        # All channels in this synthetic epoch share one transmit time. Mirror
+        # firmware's wrap-safe corrected RF-sample difference. The fractional
+        # code residue removes the 61 ns integer-sample quantization; the
+        # separately captured 125 MHz counter is used only for PPS epoch mapping.
+        observations_m = (
+            corrected_abs - corrected_abs[reference_index]
+        ) * (C / RF_HZ)
         observations_m += C * np.asarray(
             [measurement.satellite_clock_s for measurement in measurements]
         )
@@ -711,7 +714,6 @@ class ReceiverEpochSimulator:
         gps_reference_estimate = (
             self.gps_tow_start_s
             + transmit_true_s
-            + START_OFFSET_S
             - wls.clock_bias_m / C
         )
         gps_reference_true = self.gps_tow_start_s + reference.arrival_true_s