Delegate smart_merge to the tailwind_merge gem

[jeffdlange]

Summary

Replaces the hand-rolled Tailwind conflict-resolution logic in smart_merge with a delegation to gjtorikian/tailwind_merge (the Ruby port of tailwind-merge). The public API is unchanged.

• lib/view_component_css_dsl.rb: -165 lines net. Dropped SPACING_REGEX, SPACING_AXIS_MAP, BORDER_REGEX, CATEGORIES, KNOWN_MODIFIERS, MODIFIER_PATTERNS, plus spacing_info, detect_category, extract_modifier_prefix, known_modifier?. Added one module-level MERGER = TailwindMerge::Merger.new shared across the process.
• view_component_css_dsl.gemspec: added tailwind_merge ~> 1.0 dependency.
• CHANGELOG.md: noted under Changed (broader coverage) and Breaking (hidden semantics).
• README.md: noted the delegation and added a JS-toggle visibility subsection pointing to the HTML5 hidden attribute pattern.
• TODO.md: removed the resolved investigation item.

Why

• Coverage: smart_merge only categorized ~14 utility groups. tailwind_merge covers every Tailwind utility group and tracks upstream Tailwind releases (1.5.0 ships TW 4.3 support, May 2026). Groups now correctly merged that weren't before: shadow-*, ring-*, gap-*, space-*, divide-*, z-*, opacity-*, leading-*, tracking-*, transition-*, scale-*, rotate-*, translate-*, blur-*, plus arbitrary-value negative spacing and stacked !important markers.
• Parity confirmed: 16/16 spacing/border shorthand-vs-longhand cases produce identical output to the old logic (including responsive/state/stacked modifier prefixes).
• Performance: measured 28–68× faster across realistic workloads (cache-miss path, repeated-input path, multi-arg form). The library's existing _css_cache/_css_merge_cache sit above this and continue to work unchanged.
• Maintenance: stops reimplementing tailwind-merge from scratch every time a new utility group ships.

Breaking change

hidden is now treated as part of the display group, so "flex hidden" collapses to "hidden" (matching upstream tailwind-merge). The previous carve-out preserved both classes for JS-toggle patterns. The supported replacement is the HTML5 hidden attribute via the existing attribute DSL — :hidden is already in HTML_ATTR_KEYS, so callers can pass hidden: true or components can declare attribute hidden: -> { ... }. JS then toggles with element.toggleAttribute('hidden') instead of classList.toggle('hidden').

Test plan

• bundle exec rspec — 139 examples, 0 failures (no spec changes needed)
• bundle exec standardrb — clean
• Direct parity test against old smart_merge on 40+ cases (spacing/border/modifiers/arbitrary/important/negative) — all spacing cases identical, all coverage-gap cases newly handled correctly
• Forge audit (already done out-of-band): 3 sites rely on the old hidden carve-out and will be migrated separately after this lands

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