miner: give verification failures a dedicated exit code

[aryansk]

Fixes #10337

Make verification_failed a first-class attempt outcome and return exit code 12 for it instead of the generic unexpected-error code. Document the terminal outcome mapping beside the switch and add focused CLI regression coverage.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.81%. Comparing base (f665d94) to head (2b3529c).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #10362       +/-   ##
===========================================
- Coverage   92.31%   80.81%   -11.51%     
===========================================
  Files         939      285      -654     
  Lines      114824    59499    -55325     
  Branches    27731     8863    -18868     
===========================================
- Hits       105997    48082    -57915     
- Misses       7527    11123     +3596     
+ Partials     1300      294     -1006     

Flag	Coverage Δ
backend	100.00% <100.00%> (+4.27%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
packages/loopover-miner/lib/attempt-cli.ts	100.00% <100.00%> (ø)
packages/loopover-miner/lib/attempt-runner.ts	100.00% <100.00%> (ø)

... and 786 files with indirect coverage changes