feat: export is_decoding_completed_for_positioning for PVT readiness

[giove-a]

Summary

Exports is_decoding_completed_for_positioning so downstream consumers (e.g.
PositionVelocityTime.jl) can gate use of a satellite in a fix without reaching
into decoder internals. It pairs with the already-exported is_sat_healthy:

if is_decoding_completed_for_positioning(state) && is_sat_healthy(state)
    # satellite is ready and healthy for PVT
end

Changes

• Export is_decoding_completed_for_positioning.
• New GNSSDecoderState method that forwards to the validated data field
  (not raw_data), mirroring is_sat_healthy(state). It only returns true
  once the required message set has passed CRC/parity and the cross-subframe
  issue-of-data consistency check that promotes raw_data → data.
• Docstring recording the guarantee and the deliberate non-guarantees.
• API reference entry (required by checkdocs = :exports).
• Tests covering the new state-forwarding method (fresh decoder → false;
  fully decoded state → true).

Guarantees

• Covers the core per-satellite PVT requirements: time-of-week, full ephemeris,
  SV clock polynomial (plus week number / single-band group delay where those
  signals fold them into the required set).
• Verified across all six signals (L1 C/A, L1C-D, L5I, L2CM, E1B, E5a): whenever
  the predicate is true, the exact health field is_sat_healthy reads has
  already been decoded — so the two checks compose safely without a separate
  nothing guard.

Deliberate non-guarantees (documented in the docstring)

A true is a necessary condition, not a blanket "no further thinking":

1. Ephemeris freshness — presence is checked, not age; the decoder has no
   notion of "now", so consumers must still reject ephemerides outside their fit
   interval.
2. Second-order corrections — T_GD/ISC_* beyond the required band,
   Klobuchar ionosphere, and UTC params are intentionally excluded (broadcast
   far less often); apply when present, treat nothing as zero.
3. Alert flag — is_sat_healthy reflects broadcast health bits only. GPS
   L1 C/A and CNAV (L5I/L2C) also carry a per-message alert flag
   (state.data.alert_flag) that a strict-integrity receiver may want to honour
   separately. (Galileo's equivalent — Data Validity Status — is already folded
   into is_sat_healthy; L1C-D has no alert flag.)

Testing

• Full suite passes (2495 tests), including new assertions on the exported
  state method.
• Docs build passes locally (checkdocs, doctests, cross-references).
• JuliaFormatter 2.8.5 leaves src/test unchanged.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.15%. Comparing base (9fa1c6b) to head (07b01b4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #74   +/-   ##
=======================================
  Coverage   98.15%   98.15%           
=======================================
  Files          14       14           
  Lines        1519     1520    +1     
=======================================
+ Hits         1491     1492    +1     
  Misses         28       28           

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