feat!: enrich PVT solution with IFB reference bands, units, and course over ground

Summary

Enriches PVTSolution with an explicit inter-frequency-bias representation, a
course-over-ground field, and Unitful units on its metre-valued fields, and
tightens the warm-start seeding of the inter-frequency biases. Also fixes a
code-phase double-count in calc_uncorrected_time for jointly tracked satellites.

This is a breaking release (4.0.0): the PVTSolution field layout and several
field types change.

What changed

Inter-frequency biases carry their reference band (InterFrequencyBias)

inter_frequency_biases is now Dict{Symbol, InterFrequencyBias} instead of
Dict{Symbol, Float64}. Each InterFrequencyBias bundles:

• value::typeof(1.0m) — the bias, in metres, relative to reference;
• reference::Symbol — the band its bias is measured against (the anchor whose
  delay is folded into the clock biases).

The reference band is chosen per coverage component and threaded from
band_ifb_layout → decide_bias_layout → calc_pvt, so each reported bias's
anchor is explicit rather than implicit. A disconnected constellation can in
principle report distinct references per band.

Course over ground

Adds course_over_ground::typeof(1.0°): the azimuth of the velocity vector in the
local ENU frame at position, clockwise from true North, wrapped to [0, 360)°
(the GNSS COG convention). It is the velocity-derived direction of motion, not
vehicle heading — a single-antenna receiver cannot observe heading. 0° when the
horizontal velocity is zero. Computed by the new calc_course_over_ground helper.

Units on metre-valued fields

time_correction, the inter_system_biases values, InterFrequencyBias.value, and
SatInfo.residual are now typeof(1.0m) Unitful quantities (joining the
degree-valued course_over_ground). position/velocity stay plain ECEF (Geodesy
interop) and relative_clock_drift stays dimensionless. The least-squares solver
still runs on plain Float64; units are attached only at the output boundary and
stripped at the warm-start seed, so there is no runtime cost (a Quantity{Float64}
is bit-identical to a Float64).

Reference-aware IFB warm start

Each IFB column is seeded from the previous solution only when that band's reference
band is unchanged. On a reference change the stored value refers to a different
anchor, so the column is seeded at 0 instead. The IFB enters the design linearly,
so this affects convergence, not the converged estimate.

Fix: calc_uncorrected_time code-phase double-count

For jointly tracked satellites, reduce the shared code phase modulo one data symbol
so the whole-symbol part already counted by the bit term is not added twice (a no-op
for single-signal sats). E5a/L2C test fixtures updated to build realistic
measurements (whole data symbols in the bit count, residual chips in the code phase).

Breaking changes / migration

• PVTSolution gains course_over_ground, changing the positional-constructor
  arity/order and the field layout — update positional construction and
  reflection-based code. Keyword construction is unaffected.
• time_correction, inter_system_biases values, and SatInfo.residual are now
  Unitful typeof(1.0m) quantities, not bare Float64 metres. Code doing
  arithmetic on them must handle units, e.g. ustrip(u"m", pvt.time_correction).
• inter_frequency_biases is now Dict{Symbol, InterFrequencyBias}. Read
  pvt.inter_frequency_biases[:L5].value (a typeof(1.0m)) and .reference
  (the anchor band) instead of a bare Float64.

Testing

• Full test suite passes, including the inter-frequency-bias set (unit assertions
  updated to compare against Unitful quantities) and the new
  calc_course_over_ground tests.

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