feat(galileo_e5a): back-patch almanac reference epochs from a later WT5

[giove-a]

What

Galileo F/NAV broadcasts each almanac's reference epoch (WN_a/t_0a) only
in Word Type 5, but a WT6 can carry an otherwise-complete SVID-3. If the
paired WT5 is missed — mid-stream acquisition, or an IOD cutover that lands a
WT6 without its WT5 — that SVID-3 record is stored without its epoch and, until
now, stayed permanently incomplete.

This PR adds backpatch_almanac_epochs: since WN_a/t_0a are shared by every
almanac of a given IOD_a, a later WT5 back-fills them into any stranded record.
A one-shot WT6 orbit therefore becomes usable even if that WT6 never reappears —
something a "store only when complete" policy could never recover, because the
WT5 alone carries none of SVID-3's orbital data.

Details

• Epoch back-patch (headline): on every WT5, complete any earlier record
  with a matching IOD_a that is still missing its epoch. Copy-on-write — the
  dictionary shared with raw_data is never mutated in place.
• Decode-everything policy retained & documented: a WT6 SVID-3 without its
  epoch is still stored (WN_a/t_0a = nothing) rather than discarded. The
  almanacs field docstring now states plainly that a record may be incomplete
  (any field can be nothing, the epoch in particular) so callers check the
  fields they need before use.
• Per-page scratch reuse: hoist soft_page (488 Float32) and crc_bits
  (238 Bool) into GalileoE5aCache instead of reallocating them per synced
  page, mirroring the CNAV preallocation convention. Consistency cleanup — runs
  once per page, so runtime impact is small.

Testing

• New testset for the partial-record case: WT6 without its WT5 stores the orbit
  with WN_a/t_0a = nothing.
• New testset for the back-patch: a later WT5 completes a stranded SVID-21 whose
  only WT6 was seen once.
• Full suite green (2457 tests).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.15%. Comparing base (5f3a69c) to head (e7b3f75).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   98.13%   98.15%   +0.01%     
==========================================
  Files          14       14              
  Lines        1505     1515      +10     
==========================================
+ Hits         1477     1487      +10     
  Misses         28       28              

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