refactor: compact kernel headers below 7000-line budget

[konard]

Summary

Fixes #360.

• Compact the tracked include/pmm/** kernel subtree from 8794 to 6947 lines, below the new blocking 7000 line budget.
• Preserve include anchor headings while removing blank-line bulk and consolidating repeated template boilerplate in block field traits and small PMM accessors.
• Align scripts/check-repo-guard-rollout.sh with the same 7000 line size rule already present in repo-policy.json.
• Regenerate single_include/pmm/pmm.h from the compacted modular headers.

Change Contract

change_type: chore
scope:
  - include/pmm/**
  - single_include/pmm/pmm.h
  - scripts/check-repo-guard-rollout.sh
  - tests/test_issue352_kernel_subtree_size.cpp
  - changelog.d/20260426_140000_issue360_kernel_compaction.md
budgets:
  max_new_files: 1
  max_new_docs: 0
  max_net_added_lines: 130
must_touch:
  - include/pmm/block_field.h
  - include/pmm/persist_memory_manager.h
  - include/pmm/forest_domain_mixin.inc
  - scripts/check-repo-guard-rollout.sh
  - tests/test_issue352_kernel_subtree_size.cpp
must_not_touch:
  - docs/**
  - README.md
  - CHANGELOG.md
  - .github/**
  - demo/**
  - examples/**
  - benchmarks/**
expected_effects:
  - Tracked include/pmm kernel source is below the 7000-line size rule.
  - Existing include anchor headings remain valid.
  - The rollout checker validates the same aggregate threshold as repo-policy.json.
  - Single-header artifacts are refreshed from the compacted modular headers.

Reproduction

• Before the fix, ctest --test-dir build -R test_issue352_kernel_subtree_size --output-on-failure failed with 8794 <= 7000.
• After the fix, git ls-files 'include/pmm/**' | xargs wc -l reports 6947 total.

Verification

• cmake --build build --target test_issue352_kernel_subtree_size
• ctest --test-dir build -R test_issue352_kernel_subtree_size --output-on-failure
• cmake --build build
• ctest --test-dir build --output-on-failure (94/94 passed)
• bash scripts/check-repo-guard-rollout.sh
• bash scripts/check-docs-consistency.sh
• GITHUB_BASE_REF=main bash scripts/check-changelog-fragment.sh
• tracked clang-format check matching CI exclusions
• tracked source file-size check (<=1500 lines)
• bash scripts/generate-single-headers.sh --output-dir /tmp/generated-pmm-issue360 plus diff -qr single_include/pmm /tmp/generated-pmm-issue360
• git diff --check HEAD~1..HEAD

Local note: cppcheck is not installed in this runner, so local cppcheck was not run.

[konard]

Solution summary

Implemented and pushed the issue 360 fix.

PR 361 is updated and ready for review: #361

Key changes:

• Reduced tracked include/pmm/** from 8794 to 6947 lines, below the 7000 budget.
• Preserved include anchor headings; test_issue354_include_anchors passes.
• Consolidated repeated block-field/accessor template boilerplate.
• Aligned scripts/check-repo-guard-rollout.sh with the 7000 rule.
• Regenerated single_include/pmm/pmm.h.
• Removed the scaffold .gitkeep.

Verification:

• Local ctest --test-dir build --output-on-failure: 94/94 passed.
• Local size regression now passes: 6947 <= 7000.
• GitHub CI for PR 361 is fully green, including cppcheck, matrix builds, sanitizers, single-header freshness, docs, changelog, and repo-guard.
• Worktree is clean.

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[konard]

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[konard]

✅ Ready to merge

This pull request is now ready to be merged:

• All CI checks have passed
• No merge conflicts
• No pending changes

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