fix(ci): preserve recipe failure diagnostics

[martin-kolinek]

🤖 Fixes CI failures whose useful recipe output could be obscured by a later synthetic failure step. This was exposed by the Windows coverage gate in run 33158834495, where cargo-gamma-rt measured 90.7% against a 90.8% threshold but the failed step did not present that diagnosis clearly.

Changes

• propagate Just's captured exit status from the same step that streams the complete recipe output
• remove the synthetic follow-up failure step
• add exact coverage counts, uncovered source ranges, and explicit no-data diagnostics to cargo-coverage-gate output
• run PR and scheduled Codecov uploads under always() when LCOV reports exist
• retain the Windows ARM exclusion and avoid empty uploads
• update generated workflows, design documentation, tests, and snapshots

Validation

• cargo-anvil GitHub artifact unit tests
• cargo-anvil generated-workflow snapshot tests
• actionlint validation for emitted workflows
• cargo-anvil Clippy with warnings denied
• cargo-coverage-gate unit and CLI tests
• formatting, generated README, and spelling checks

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.9%. Comparing base (48c5311) to head (0d0e8b7).

❌ Your project status has failed because the head coverage (99.9%) is below the target coverage (100.0%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #110    +/-   ##
======================================
  Coverage   99.9%   99.9%            
======================================
  Files        135     135            
  Lines      17470   17607   +137     
======================================
+ Hits       17469   17606   +137     
  Misses         1       1            

Flag	Coverage Δ
linux	99.9% <100.0%> (?)
linux-arm	99.9% <100.0%> (?)
windows	99.9% <100.0%> (?)

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