Combine satellites from multiple GNSS in `calc_pvt` and add the modern
GPS and Galileo civil signals.

Multi-GNSS:
- Estimate one receiver clock bias per GNSS time system; the state vector
  becomes [x, y, z, tc_1, …, tc_M] and a fix needs n >= 3 + M satellites.
  Time-system grouping uses GNSSSignals.get_time_system(signal) and the
  GPST()/GST() TimeSystem singletons.
- GGTO fallback: when the independent inter-system-bias solve is
  under-determined but GPS is present and a Galileo satellite carries the
  Galileo-GPS Time Offset, collapse the Galileo clock onto GPS via the
  broadcast offset (range_offset = -c * GGTO), enabling a 4-satellite
  GPS+Galileo fix.
- `PVTSolution` gains `reference_system` and `inter_system_biases`, keyed by
  GNSSSignals.TimeSystem.

Inter-frequency bias:
- When satellites are processed on more than one frequency band, estimate
  one receiver inter-frequency bias (IFB) per band beyond a reference band —
  the differential RF-chain hardware delay — shared across constellations on
  that band (GPS L1/L1C/Galileo E1 -> :L1; GPS L5/Galileo E5a -> :L5;
  GPS L2C -> :L2; see `GNSSSignals.get_band_id`). The state vector extends to
  [x, y, z, tc_1…tc_M, ifb_1…ifb_B] and a fix needs n >= 3 + M + B satellites.
- `PVTSolution` gains `inter_frequency_biases` (relative to the reference
  band; empty for single-band fixes). Velocity is unaffected (a constant
  bias, no drift term); the IFB composes with the GGTO clock collapse.
- Observability: the per-system clock and per-band IFB columns form the
  incidence matrix of the (constellation × band) coverage graph. When that
  graph is disconnected — constellations occupying disjoint bands — those
  columns are collinear and the independent layout is rank-deficient, which
  previously yielded degenerate clock/IFB estimates. The layout now detects
  this and falls back: collapse the Galileo clock onto GPS via the GGTO when
  available, otherwise estimate the inter-system bias and IFB as a single
  common parameter (they can no longer be separated).

New signals:
- GPS L5 (CNAV) and L1C (CNAV-2): quasi-Keplerian ephemerides
  (A_REF + ΔA, Ω̇_REF + ΔΩ̇, rate terms) via `orbital_elements`, full-week
  number, ITOW/TOI time-of-week reconstruction for L1C-D.
- GPS L2C (CNAV on the new L2 band): shares the CNAV `GPSCNAVData` container
  with L5-I — the signal is fixed by the decoder's constants type, not the
  data type — so it reuses the CNAV orbit/clock/week/Klobuchar path; it ranges
  on the L2 CM data component with the ISC_L2C inter-signal correction, and
  introduces the L2 band into the inter-frequency-bias layout.
- Galileo E5a (F/NAV): shares the Galileo ephemeris/clock/GGTO/NTCM-G layout
  with E1B; applies the E1-E5a broadcast group delay.
- Galileo E1C, GPS L1C-P, GPS L5-Q and GPS L2 CL pilots are not separate
  measurements: with joint same-band tracking the band yields one pseudorange,
  so pilots aid tracking upstream only.

Dispatch is grouped with `GalileoNavData`, `GPSNavData` and `GPSModernNavData`
type aliases; Klobuchar is extended to all GPS civil signals and NTCM-G to
both Galileo messages.

Group delay / inter-signal correction is selected by the *ranging* signal
(`SatelliteState.system`) with values read from the decoder's message, so a
band's pseudorange may be generated on a pilot (e.g. GPS L1C-P) while the
ephemeris/clock come from the data-component decoder (CNAV-2 → ISC_L1CP vs
ISC_L1CD vs ISC_L1CA; CNAV → ISC_L5I5/ISC_L5Q5 vs the single ISC_L2C
shared by CM and CL). The transmit-time bit-count
term takes its symbol rate from GNSSSignals.get_data_frequency on the decoder
state (GNSSDecoder 3.6), not the tracked signal, so a pilot's 0 Hz data rate is
never divided by, and CNAV picks the right rate per signal (L5-I 100 Hz vs
L2C-M 50 Hz) via the decoder's constants type. Galileo's broadcast group delay
stays per band (decoder-determined).

Solver and outputs:
- `sats` is keyed by (signal, PRN) in a `Dictionary` so the same PRN can
  appear across constellations and bands; new `get_sat_info` accessor.
- `SatInfo` carries the post-fit pseudorange residual.
- DOP is rotated into the local ENU frame for HDOP/VDOP and uses a
  non-throwing Cholesky; TDOP reports the reference system's clock.
- Velocity/clock-drift solved via allocation-free 4x4 normal equations with
  a single common receiver clock drift across all systems.
- calc_pvt skips the atmospheric-delay prediction (a per-satellite geodetic +
  ENU pass) when neither ionospheric nor tropospheric correction is active, and
  builds the satellite-position matrix with `stack` in a single pass.

Type-level getters and time scale (GNSSSignals 3.3):
- The ionospheric/velocity carrier-frequency reads take constants from the
  signal *type*, never a constructed instance:
  constructing a signal builds its full code table (~120 KB GPS L1CA, ~620 KB
  Galileo E1B) and this ran once per satellite, so it cut calc_pvt allocations
  ~97% (GPS) / ~99% (Galileo).
- The time-scale origin comes from GNSSSignals.get_system_start_time (UTC),
  converted to a TAIEpoch via AstroTime's leap-aware from_utc
  (system_start_epoch); the hardcoded epochs and leap-second literals are gone.
  system_start_epoch lands on an integer TAI second for GPS/Galileo, so the
  reported-time build assumes start_time.fraction == 0 (noted inline).
- Per-signal and per-band classification uses GNSSSignals.get_signal_id
  (:GPSL1CA, …) and GNSSSignals.get_band_id (:L1, …) on each satellite's
  signal, instead of package-local helpers (GNSSSignals 3.3).
- The nav-message symbol rate for the transmit-time bit count comes from
  GNSSSignals.get_data_frequency on the decoder state (GNSSDecoder 3.6),
  replacing the six package-local nav_data_frequency methods.

Compat: GNSSDecoder 3.6, GNSSSignals 3.3, add Dictionaries, drop Statistics.

Tests: add cnav.jl, dop.jl, galileo_e5a.jl, gps_l2c.jl and
inter_frequency_bias.jl; the satellite fixtures move to test/fixtures.jl,
shared with benchmark/fixtures.jl (which keeps a GNSSDecoder major-version
shim so AirspeedVelocity can run the script against the base revision); cover combined GPS+Galileo fixes, the GGTO fallback,
(signal, PRN) keying, per-satellite residuals, ranging-signal-selected group
delay (incl. pilot-referenced ranges and ISC_L2C), per-band inter-frequency-bias
estimation (incl. GPS L1+L2), and the disjoint-band fallback.

BREAKING CHANGE: `PVTSolution.sats` is now a
`Dictionary{Tuple{Symbol,Int},SatInfo}` (was `Dict{Int,SatInfo}`); index it
with `get_sat_info(pvt, signal, prn)`. `SatInfo` gains a `residual` field and
`calc_DOP` takes the user position and primary clock index. The
`get_gdop`/`get_pdop`/`get_hdop`/`get_vdop`/`get_tdop` accessors are removed;
read DOP from the `dop` field instead (e.g. `pvt.dop.GDOP`).
`get_num_used_sats` is removed: with `sats` keyed by (signal, PRN),
`length(pvt.sats)` counts measurements, not satellites.
`get_frequency_offset` is removed: compute it inline as
`pvt.relative_clock_drift * base_frequency`.
`PVTSolution.reference_system` and the `inter_system_biases` keys are now
`GNSSSignals.TimeSystem` values (GPST()/GST()), not :GPS/:Galileo symbols.
Requires GNSSSignals 3.3 and GNSSDecoder 3.6.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>