MDS036: add per-heading fixtures and correct plan 51 wording

[jeduden]

Follow-up to #143 addressing the last two Copilot review comments that arrived after that PR entered the merge queue.

Summary

• Add good/per-heading.md and bad/per-heading.md fixtures so the regex-based override path is exercised end-to-end through fixture YAML decoding (not just unit tests).
• Correct plan 51 task 3 text from "by lines or tokens" to "by lines" — the implementation is line-based only.

Test plan

• go test ./internal/integration/... ./internal/rules/maxsectionlength/...
• go tool golangci-lint run ./...
• go run ./cmd/mdsmith check .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.76%. Comparing base (fef3ac3) to head (79aa5bf).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #149      +/-   ##
==========================================
+ Coverage   86.70%   86.76%   +0.05%     
==========================================
  Files          95       96       +1     
  Lines       10387    10446      +59     
==========================================
+ Hits         9006     9063      +57     
- Misses        899      900       +1     
- Partials      482      483       +1     

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[Copilot]

Pull request overview

Follow-up to #143 that adds end-to-end integration fixtures for MDS036’s per-heading override path and corrects plan 51 wording to match the line-based implementation.

Changes:

• Add good/per-heading.md and bad/per-heading.md fixtures to exercise regex-based per-heading settings via YAML front matter decoding.
• Update plan 51 task wording from “by lines or tokens” to “by lines”.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
plan/51_section-level-size-limits.md	Aligns plan wording with the implemented line-based section length counting.
internal/rules/MDS036-max-section-length/good/per-heading.md	Adds a passing fixture verifying per-heading limits apply only to matching headings with max: 0 disabling the global default.
internal/rules/MDS036-max-section-length/bad/per-heading.md	Adds a failing fixture asserting the expected diagnostic for an over-limit section matched by a per-heading regex.

[jeduden]

🟢 Merge Queue — picked up

This PR is in the queue and will be batched with other queue-labelled PRs.

Next: No action needed — you'll get another comment when CI starts on the batch. View merge queue run.

[jeduden]

🔵 Merge Queue — CI running

Merged into batch branch merge-queue/batch-149-1776700029. View CI run.

Next: No action needed — you'll be notified when CI completes.

[jeduden]

✅ Merge Queue — merged

This PR landed on main via commit 89f05bb. CI run that validated the merge.

Next: Done — nothing more to do here.