Extend visibility-aware search and fix dual-band acquisition scheduling

Visibility-aware acquisition search (GNSS-SDR.enable_visibility_aware_search):
- Classify GLONASS and QZSS satellites too. GLONASS positions come from the
  GNAV almanac (new Glonass_Gnav_Almanac::satellite_position with the
  standard perturbation model), so the search order benefits before any
  GLONASS ephemeris is decoded.
- PVT publishes a lock-free navigation snapshot (ephemeris, almanac, UTC
  models per system) that the classifier reads instead of locking the PVT
  maps on every recompute.
- Recompute on new navigation data, on position changes larger than
  visibility_recompute_position_threshold_m, every
  visibility_recompute_interval_s, and when the almanac ages out; fix the
  periodic and position-triggered recomputes that never fired, and the
  off-by-one that excluded satellites in [mask, mask+1) degrees.
- Enter the start-up standby only when the feature is enabled, so the
  default search order is untouched.

Secondary-frequency acquisition assistance:
- Do not start an acquisition on the PRN 0 placeholder left on assisted
  channels at connect time; it threw inside the control thread and left the
  flowgraph running headless with the secondary band never acquired.
- Match GLONASS assistance by FDMA frequency channel: the search pool keeps
  one slot per channel while tracked L1 channels report the decoded slot, so
  L2 was never picked.
- Assist from locked channels before their telemetry is decoded: the
  observables status now reports tracking-only channels and the flowgraph
  keeps a separate tracking map for the assisted search.

docs/CHANGELOG.md

@@ -132,6 +132,25 @@ All notable changes to GNSS-SDR will be documented in this file.
   the generic rule
   `doppler(col) = -doppler_max + doppler_center + doppler_step * col` decodes
   both full and narrowed grids. Contributed by @joebre.
+- Added an optional visibility-aware acquisition search, enabled with
+  `GNSS-SDR.enable_visibility_aware_search=true` (default `false`, which leaves
+  the existing search order untouched). Once a receiver position is available,
+  either from a fix or from `GNSS-SDR.AGNSS_ref_location`, satellites are
+  continuously classified as visible, excluded (elevation at or below
+  `GNSS-SDR.search_elevation_mask`, default 0 degrees, or flagged unhealthy), or
+  not yet known, using the freshest ephemeris or almanac decoded for GPS,
+  Galileo, BeiDou, GLONASS, and QZSS. Idle channels then favor visible
+  satellites over unknown ones, at the ratio given by
+  `GNSS-SDR.visible_vs_mayvisible_search_ratio` (default 3), and skip known
+  excluded ones, so less CPU is spent acquiring satellites that are below the
+  horizon. The classification is refreshed whenever new navigation data arrives,
+  when the receiver moves more than
+  `GNSS-SDR.visibility_recompute_position_threshold_m` (default 1000 m), every
+  `GNSS-SDR.visibility_recompute_interval_s` (default 120 s), and when almanac
+  data becomes older than `GNSS-SDR.visibility_almanac_max_age_s` (default 3
+  days). A satellite that is already being tracked is never released because of
+  this classification, and `PVT.elevation_mask` still decides which observations
+  enter the navigation solution. Contributed by @joebre.
 
 ### Improvements in Interoperability:
 
@@ -333,6 +352,14 @@ All notable changes to GNSS-SDR will be documented in this file.
   metadata in satellite status, and the NMEA printer emits the strongest
   available C/N0 with the corresponding NMEA signal identifier. Contributed by
   @vladisslav2011.
+- The custom output stream defined by `monitor_pvt.proto` now includes a
+  `tracked_satellites` list. Each entry reports one tracked signal (`system`,
+  `prn`, `signal`), its `azimuth_deg` and `elevation_deg`, whether it was
+  `combined` with another signal of the same satellite (e.g., the Galileo E1+E5a
+  ionosphere-free combination), and a `used` flag telling whether it contributed
+  to the reported fix. Satellites that were tracked but left out of the solution
+  (below `PVT.elevation_mask`, or excluded by RAIM) are listed with
+  `used = false`. Contributed by @joebre.
 
 ### Improvements in Maintainability: