refactor(partition): ♻️ name the node-mask regimes instead of a bare bool

[diagonal-hamiltonian]

🤖 AI text below 🤖

Summary

Follow-up to #249, which shipped the cgroup placement fix with a bool mask_is_private parameter.
This renames that parameter to enum class NodeMask { Shared, PerRank }. No behaviour change.

partition_cpusets(n, gi, gc, true) does not say at the call site which regime true means, and
the safe default is spelled false. The enum names both, and a third launcher regime would be a
new enumerator rather than a second bool.

Changes

• enum class NodeMask { Shared, PerRank } in CpuTopology.h.
• partition_cpusets's fourth parameter becomes NodeMask mask = NodeMask::Shared.
• PartitionGroup::node_mask_private_ becomes NodeMask node_mask_; classify_node_masks_
  converts masks_are_pairwise_disjoint's bool at the call site. The predicate keeps its bool
  return -- it answers a question about masks, not about policy.
• Test call sites lose their /*mask_is_private=*/ argument comments, which the enum value replaces.

Every assertion and every test-case name is unchanged, and grep -rn mask_is_private cpp/ is empty.

Checklist

• Tests added or updated to cover the changes
• Documentation updated (docstrings, docs/, CONTRIBUTING.md) if needed
• CHANGELOG / release notes updated if applicable

AI/LLM disclosure

• I did not use LLM tooling, or used it only privately for ideation
• I used the following tool to help write this PR description: Claude Code
• I used the following tool to generate or modify code: Claude Code

[github-actions]

Docs preview: https://pr-252.monoprop-docs.pages.dev

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.70%. Comparing base (1a4177e) to head (313a891).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #252   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          14       14           
  Lines         742      742           
  Branches       98       98           
=======================================
  Hits          725      725           
  Misses         12       12           
  Partials        5        5           

Flag	Coverage Δ
cpp	97.70% <ø> (ø)

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

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud