feat: Galileo E5a F/NAV decoder

[giove-a]

Summary

Adds a decoder for the Galileo Open Service F/NAV message broadcast on the
E5a-I data component (50 sps). It recovers ephemeris, clock correction,
ionospheric correction, BGD, signal health / data validity, GST-UTC and GST-GPS
conversion, and the word-type-5/6 split-almanac chain — per OS SIS ICD v2.2 §5.1
(word types 1–6).

Usage mirrors the existing decoders:

state = GalileoE5aDecoderState(prn)            # or GNSSDecoderState(GalileoE5aI(), prn)
state = decode(state, soft_symbols, n)
is_sat_healthy(state) && # state.data ready for positioning

What's in here

• src/galileo/e5a.jl — F/NAV decoder: page sync, FEC, CRC, and word-type
  1–6 field parsing, plus the SVID-2 split-almanac stitching across WT5/WT6.
• src/galileo/galileo.jl (new) — definitions shared by every Galileo
  signal, factored out of e1b.jl: the SignalHealth / DataValidityStatus
  enums, GalileoAlmanac, and the common galileo_viterbi FEC primitive.
• src/gnss.jl — hoists the package-wide physical constants (GNSS_PI,
  SPEED_OF_LIGHT, EARTH_ROTATION_RATE) to a single source of truth; GPS
  decoders now reference them (values unchanged).

The FEC chain reuses the shared Galileo K=7 NSC Viterbi (G1=0o171, G2=0o133 with
G2 inverted): 12-symbol sync detection → 61×8 block deinterleave → G2 sign-flip
on the soft LLRs → AFF3CT Viterbi (K=238, N=488) → CRC-24Q over the 238-bit page.
Consumes soft symbols end-to-end like L1 C/A and E1B, so the public decode API
is unchanged.

Testing

test/galileo_e5a.jl drives the full decode path against a Spirent PRN 21
capture (152 F/NAV pages). Field values are checked against the independent
Spirent ground-truth dump, so a layout mistake can't be masked by a shared
encode/decode bug. Covered: FEC round-trip, CRC validity, the full
ephemeris/clock frame, the WT5/WT6 split-almanac stitching, the 180° polarity
ambiguity, and reset semantics. Full suite: 2043 / 2043 pass.

Dependency / CI note

Requires GNSSSignals ≥ 2.3 (compat bumped from 2.2), which adds the
GalileoE5aI / GalileoE5aQ signal types (JuliaGNSS/GNSSSignals.jl#83).

⚠️ At time of opening, GNSSSignals 2.3.0 is merged to master but not yet
registered in General (registry max is 2.2.2). CI will stay red until
2.3.0 is registered — the "2.3" compat can't resolve before then, so the
failing jobs can simply be re-run once it lands. No further changes are needed
here.

Compatibility

Non-breaking — purely additive (new GalileoE5aDecoderState export). The
e1b.jl / gnss.jl refactor relocates code without changing any public symbol,
signature, or constant value. Suggest a minor version bump (3.0.0 → 3.1.0).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.13%. Comparing base (6c0fa32) to head (d12a05b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   97.67%   98.13%   +0.45%     
==========================================
  Files          10       12       +2     
  Lines        1336     1499     +163     
==========================================
+ Hits         1305     1471     +166     
+ Misses         31       28       -3     

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