✨ PwOutput: Parse k-points from stdout as fallback

[mbercx]

The XML is the structured source of truth for the k-point grid, but it is common to discard the QE outdir after a run, or to overwrite it with a follow-up nscf/bands calculation. With only stdout on disk, the three k-point outputs (number_of_k_points, k_points_weights, k_points_cartesian) were previously unreachable, even though the same information is printed verbatim in the stdout cart. coord. in units 2pi/alat block. This pattern — XML preferred, stdout as a last-resort fallback — is now expected to recur for other outputs.

PwStdoutParser captures the last number of k points= block (the last one wins because vc-relax reprints after the relaxed cell is set), reads the k( i ) = ( x y z ), wk = w rows, and converts the cartesian coordinates from 2π/alat to 1/Å using the stdout lattice parameter (alat) so the units match the XML-derived path. Stdout precision is fundamentally lower than XML (5-digit alat, 7-digit k components) so the fallback values agree with the XML only to ~1e-3 in 1/Å; one of the new tests pins this tolerance.

Each of the three Specs is wrapped in Coalesce(xml, stdout) so the XML path keeps winning when present. Two trimmed fixtures (default_xml_220603, default_xml_250521) gained a reconstructed k-point block — built from their own XML at fixture-creation time — so every default_xml_* snapshot now exercises the stdout fallback.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.40%. Comparing base (d0f8124) to head (88efa57).
⚠️ Report is 131 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
- Coverage   89.77%   88.40%   -1.37%     
==========================================
  Files          11       23      +12     
  Lines         489     1087     +598     
==========================================
+ Hits          439      961     +522     
- Misses         50      126      +76     

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