[Havit.Bootstrap] Initial setup: consume Bootstrap 6.0.0-alpha1 sources and update the CSS build pipeline

[crdo]

Initial infrastructure work to get Bootstrap 6 sources into the repo and building, as the foundation for all other v6 issues.

Current state (Havit.Bootstrap):

• bootstrap@5.3.8 npm devDependency, custom Sass overrides (_variables.scss, _buttons.scss, _dropdown.scss, _forms.scss, _tables.scss, bootstrap.scss)
• Build: dart-sass (pinned 1.78.0) → PostCSS (autoprefixer) → RTL (rtlcss) → clean-css minify → publish-to-blazor copies dist CSS to Havit.Blazor.Components.Web.Bootstrap/wwwroot

Tasks:

• Source intake: consumed as npm git dependency pinned to a v6-dev commit (github:twbs/bootstrap#82b040e) — re-pin to follow upstream alphas, switch to the npm package once published.
• Sass module system: scss/bootstrap.scss rewritten to @use "bootstrap/scss/bootstrap" with (...). Note: only variables reachable through the entry's @forward chain are configurable this way (_config.scss is not forwarded — config vars would need a direct @use "bootstrap/scss/config" with (...) first).
• Brand color: $blue: #009de0 override drives the --bs-blue-* scale and the primary theme. The v5 $min-contrast-ratio: 3 hack is obsolete — v6's theme system no longer uses color-contrast() (primary uses white contrast by default).
• v5 override partials (_variables.scss, _buttons.scss, _dropdown.scss, _forms.scss, _tables.scss) kept unimported for reference; porting happens per component ([HxDropdown] -> [HxMenu] #1440, [HxButton] New variant system (.btn-solid/.btn-outline/.btn-subtle/.btn-text + .theme-*) #1442, [HxCheckbox] [HxCheckboxList] [HxRadioButtonList] [HxSwitch] New checks/radios/switches markup #1453–[HxInputBaseWithInputGroups] Simplified input groups (.has-validation removed, .input-group-ignore) #1457, Breakpoint changes (lg/xl raised, xxl -> 2xl) and prefix-based responsive class syntax #1469, Verify minor components against Bootstrap 6 (carousel, pagination, progress, placeholder, spinner, scrollspy, collapse, range, floating labels) #1471).
• Toolchain updates: sass ^1.100.0; minification switched from clean-css to lightningcss (build/css-minify.mjs from upstream) — clean-css cannot handle @layer, color-mix(), oklch(); .browserslistrc taken from upstream.
• PostCSS: postcss-prefix-custom-properties (prefix bs-) added via postcss.config.mjs, mirroring the upstream build.
• RFS removed: nothing in our pipeline depended on it.
• RTL: build step removed (v6 uses CSS logical properties; upstream has no RTL build anymore); stale *.rtl.css artifacts deleted from dist and wwwroot.
• JS dist: ESM-only bundle + HxSetup/docs references → handled in Bootstrap 6 JavaScript is ESM-only (no UMD, no window.bootstrap), Popper replaced by Floating UI #1470.
• Build & publish: npm run css + publish-to-blazor produce working v6 CSS (verified: @layer cascade, --bs- prefixes, --bs-blue-500: #009de0, .menu/.dialog/.drawer rules present) in Havit.Blazor.Components.Web.Bootstrap/wwwroot. CI is unaffected (compiled CSS is committed, not built in CI). yarn.lock dropped — package-lock.json is the single lockfile.

Implemented on the v6 branch.

Related: #1468 (theme tokens), #1469 (breakpoints/prefix syntax), #1470 (ESM-only JS)

Migration: https://v6-dev--twbs-bootstrap.netlify.app/docs/6.0/migration/#sass

[crdo]

Implemented directly on the v6 integration branch (commit 661f372); the remaining JS dist task was completed via #1473. Closing — v6 will be merged to master as part of the v6.0.0 release (#1455).

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