docs(theme): bump sidebar link min-height to 24px to satisfy a11y/touch-target

[aram-devdocs]

Summary

• Adds docs/theme/css/custom.css and wires it via book.toml's additional-css.
• Bumps sidebar chapter link min-height from ~22 px to 24 px so the docs site passes Plumb's own a11y/touch-target rule (default min_height_px = 24).

Why

From the share-readiness audit (finding H33). When plumb lint https://plumb.aramhammoudeh.com was run against the deployed docs site, the stock mdBook sidebar tripped Plumb's own a11y/touch-target rule because chapter anchors render at ~22 px on common viewports — below the 24 px WCAG 2.5.8 floor. Fixing this avoids the embarrassing "linter fails its own docs site" moment.

Approach

• Targets .chapter li a (the sidebar anchors inside <ol class="chapter">) plus .chapter-item and .expanded permutations.
• Uses min-height: 24px plus padding-top: 4px / padding-bottom: 4px and display: flex; align-items: center; so the touch target is honored even when the line-height alone wouldn't reach 24 px.
• Leaves theme/head.hbs untouched so it can coexist with parallel theme work (PR 1's version-stamp).

Test plan

• mdbook build succeeds; the hashed CSS (book/docs/theme/css/custom-<hash>.css) is referenced from book/index.html.
• just validate passes on the branch.
• After deploy: plumb lint https://plumb.aramhammoudeh.com no longer reports a11y/touch-target violations on the sidebar.

[github-actions]

I have everything I need. Here is the structured review.

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Code review summary

PR: docs(theme): bump sidebar link min-height to 24px to satisfy a11y/touch-target
Author: Aram Hammoudeh
Base: main
File bucket: docs — book.toml, docs/theme/css/custom.css

No Rust source files are touched. Determinism, layering, error-handling, and unsafe invariants are out of scope.

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Blockers

#	Severity	File	Line	Issue	Suggestion
—	—	—	—	None	—

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Warnings

#	Severity	File	Line	Issue	Suggestion
—	—	—	—	None	—

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Architecture compliance

• [✅] No new unsafe outside plumb-cdp
• [✅] No new unwrap / expect / panic! in library crates
• [✅] No new println! / eprintln! outside plumb-cli
• [✅] No new wall-clock or HashMap in observable-output paths
• [✅] No new todo! / unimplemented! / dbg!

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Anti-pattern scan

Pattern	Status	Details
AI-flavored prose in docs	✅ Clean	CSS comment is technical; no "seamless / leverage / dive in"
book.toml additional-css path	✅ Correct	Path is relative to repo root where book.toml lives
display: flex on sidebar links	✅ Safe	mdBook <a> elements in .chapter contain only text ± icon; flex has no adverse nesting effect
Padding math vs. 24 px floor	✅ Passes	min-height: 24px + padding: 4px 0 + line-height: 1.5 on ~14 px base = ~29 px effective height
Directory naming ambiguity	⚠️ Note	docs/theme/css/ is unrelated to theme/ (the custom theme dir). Non-blocking — paths are independent — but a # NOTE in book.toml could help future readers. Not a blocker.

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Quality assessment

The fix is minimal and precisely scoped: two files, 26 added lines, zero deleted lines. The motivation is verified (Plumb's own a11y/touch-target rule, min_height_px = 24, was tripping on the docs site's ~22 px sidebar links). The selectors cover all four .chapter li a states that mdBook renders. The second rule (.chapter li { line-height: 1.5; }) correctly handles non-anchor rows (dividers, numbered prefixes) to maintain visual rhythm.

Commit message follows Conventional Commits (docs(theme):). No test is warranted — CSS visual correctness is outside the scope of the Rust test suite, and no infrastructure for visual regression exists.

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Scope check

• Changes match PR description: ✅ Yes — book.toml wires additional-css; docs/theme/css/custom.css delivers the fix.
• Scope creep detected: ✅ None.

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Verdict: APPROVE