feat: introduce per-constellation data supertypes

[giove-a]

Summary

Introduces per-constellation abstract supertypes for decoder data types, mirroring the equivalent refactor in GNSSSignals.jl. This lets Galileo I/NAV (E1B) and F/NAV (E5a) share their identical ephemeris/clock completeness checks via subtype dispatch instead of duplicated per-signal copies.

Two new abstract types sit between AbstractGNSSData and the concrete per-signal data types:

• AbstractGPSData <: AbstractGNSSData — GPSL1CAData, GPSL1C_DData, GPSCNAVData
• AbstractGalileoData <: AbstractGNSSData — GalileoE1BData, GalileoE5aData

Constellation membership is now encoded at each struct's definition site (the <: Abstract*Data line written anyway), so a new signal inherits the shared behaviour with nothing to remember.

What collapsed

is_ephemeris_decoded and is_clock_correction_decoded were byte-identical for E1B and E5a — the two signals carry the same orbital (t_0e, M_0, e, sqrt_A, …, C_is) and clock (t_0c, a_f0, a_f1, a_f2) field sets. They now dispatch once on AbstractGalileoData (in src/galileo/galileo.jl) instead of once per signal — 4 methods → 2.

Genuinely per-signal checks stay on the concrete types: is_health_status_decoded (E1B carries E1-B/E5b health, E5a only E5a), is_decoding_completed_for_positioning, and is_sat_healthy.

Why GPS didn't collapse equally

The GPS supertype is added for symmetry and future-proofing (a home for any GPS-wide fact that surfaces later), but there is no current GPS duplication to remove:

• The "two signals, one message" case already shares maximally. L5I and L2C broadcast the identical CNAV message and already share a single GPSCNAVData type — reuse is at 100%, nothing to collapse.
• The three distinct GPS data types use structurally different message formats. LNAV (subframe-based), CNAV-2 (subframe-2/3 pages), and CNAV (message-type based) have different field sets, so their completeness predicates are genuinely different — not near-duplicates.

Performance

No runtime or allocation impact. The change is dispatch-only: struct layouts are unchanged, abstract supertypes are a compile-time construct, and Julia specializes methods on the concrete argument type regardless of an abstract signature annotation. Verified on the collapsed method: dispatch resolves to the single AbstractGalileoData method, return type infers to concrete Bool, 0 allocations at a concrete call site, and no dynamic dispatch in the specialized body (pure inlined getfield/isnothing).

Scope

This is the decoded-data hierarchy only. A cross-package get_time_system(decoder_state) accessor was considered and deliberately left out: PositionVelocityTime.jl already derives the time system from the ranging signal (GNSSSignals.get_time_system(state.system)), which is the authoritative source, so a parallel decoder-state path would only invite drift.

Tests & docs

• New test/gnss_supertype.jl (wired into runtests.jl): asserts the type hierarchy, that every Galileo type dispatches to the single supertype method (which), and the collapsed methods' behaviour for both signals.
• docs/src/api.md documents both new supertypes.
• Full suite: 2483/2483 pass under GNSSSignals 3.2, including Aqua (no method ambiguities/piracy).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.14%. Comparing base (016c740) to head (83ee266).

Additional details and impacted files

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- Hits         1489     1485       -4     
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