feat(gpsl2c): add GPS L2C (CNAV) decoder

[giove-a]

Summary

Adds a GPS L2C civil-navigation (CNAV) decoder per IS-GPS-200N §30, on the
v2 soft-symbol API. GPS L2C broadcasts the same 300-bit CNAV message as GPS
L5I (IS-GPS-200N §30 ≡ IS-GPS-705J §20.3.3) — identical preamble, rate-1/2 K=7
continuous FEC (G1=0o171, G2=0o133), CRC-24Q, per-message-type layouts, π, and
TOW × 6. The 25 bps → 50 sps / 12-second framing is time-domain only, so the
symbol-domain decoder is unchanged (600-symbol message, 616-symbol sync window).
Cross-checked against gnss-sdr and pocketsdr, both of which drive L2C and L5
through one CNAV decoder.

What's included

• Shared GPS CNAV core extracted into src/gps/cnav.jl (FEC, window-Viterbi
  sync, CRC gate, all parse_mt*, and the data container). src/gps/l5.jl and
  the new src/gps/l2c.jl are thin signal layers.
• GPSL2CMDecoderState(prn) + GNSSDecoderState(::GPSL2CM, prn) dispatch.
  The only decode difference from L5I is is_sat_healthy, which reports the
  L2 health bit (MT10 bit 53) vs L5's bit 54 — dispatched on a parametric
  GPSCNAVConstants{S} (aliased GPSL5IConstants / GPSL2CMConstants).
• Signal-neutral type names: the shared CNAV container/records are now
  GPSCNAVData, GPSCNAVReducedAlmanac, GPSCNAVMidiAlmanac,
  GPSCNAV{Clock,Ephemeris}DifferentialCorrection, GPSCNAVIntegritySupportMessage.
  The former GPSL5I* names remain as exported deprecated aliases
  (Base.@deprecate_binding), to be removed at the next major version — so this
  is non-breaking for downstream users.
• Shared primitives relocated: UInt320 (packed-word type) and
  _merge_keyed moved from gps/l1ca.jl / gps/l1c_d.jl into src/gnss.jl, so
  the CNAV core no longer depends on sibling signal files.

Dependency

Requires GNSSSignals ≥ 2.6.0 (adds the GPSL2CM signal type);
[compat] GNSSSignals = "2.6".

Tests

• Synthetic round-trip suite (shared CNAV transmit-chain helpers in
  test/cnav_test_utils.jl, reused by the L5I and L2C tests).
• Spirent-derived golden fixture (test/data/gps_l2c_prn25_nav_bits.bin,
  PRN 25, one full 24-message broadcast cycle) whose decoded parameters match
  Spirent's own reference decode across every message type (MT10/11, 30–37,
  12/13/14/31/34/37, 15/36).
• Full suite: 2446/2446 pass; docs build clean (doctests included).

Notes for merge

Please merge squash or rebase (not a merge commit) to keep history linear
for release CI; semantic-release will minor-bump 3.1.0 → 3.2.0 from the
feat(gpsl2c) commit.

[codecov]

Codecov Report

❌ Patch coverage is 96.83099% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.13%. Comparing base (49336b7) to head (452eb2a).

Files with missing lines	Patch %	Lines
src/gps/cnav.jl	96.70%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files          12       14    +2     
  Lines        1499     1505    +6     
=======================================
+ Hits         1471     1477    +6     
  Misses         28       28           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.