Revisit per-skin module loading mechanism

[malberts]

BootstrapComponents has a per-skin module loading mechanism: each component in ComponentsDefinition.json can declare default and per-skin module groups (vector, vector-2022), resolved via ComponentLibrary::getModulesFor( $name, $skin ). The mechanism deserves a design pass for v6.

Hardcoded skin argument. HooksHandler::onParserAfterParse calls getModulesFor( $component, 'vector' ) regardless of the actual active skin. This loads Vector-specific CSS on every skin, including ones where it actively suppresses working Bootstrap behaviour (e.g. Chameleon, Medik).

Ambiguous content. The current per-skin fix files don't all earn their place:

• popover.vector-fix.css targets .popover-title, the BS3 class name; BS4+ renamed it to .popover-header. Likely dead code on every supported install.
• modal.vector-fix.css papers over the stacking-context symptom of Stack order Modal component #84; once the modal stacking root cause is fixed (Fix Modal backdrop #91) this file has nothing left to do.
• The generic vector-fix.css is real Bootstrap-vs-Vector compatibility (font-size, box-sizing); independent of any per-component concern.

Ambiguous coverage. The declared per-skin keys are default, vector, vector-2022. MonoBook, Timeless, and Chameleon receive only default, despite being commonly used MediaWiki skins with their own Bootstrap-interaction characteristics. Whether that's intentional or accidental is unclear.

For v6, decisions to make:

• Driver: active skin, skin families, opt-in registration, body-class-scoped rules, or remove the per-skin layer entirely?
• Content: which per-skin files are still needed (audit), and is the right primitive a per-skin CSS file at all, vs. body-class-gated rules in a single neutral file, vs. Sass/Bootstrap variable overrides per skin?
• Contract: which skins does BootstrapComponents formally support per-skin handling for?

[malberts]

Audit of the fix CSS files for the BS5 migration surfaced more BS3/BS4 → BS5 class-name drift in BC's CSS modules:

• modules/ext.bootstrapComponents.popover.vector-fix.css targets .popover .popover-title — BS3 class name; renamed to .popover-header in BS4+.
• modules/ext.bootstrapComponents.carousel.fix.css targets .carousel-inner > .item — BS3 class name; renamed to .carousel-item in BS4+.
• modules/ext.bootstrapComponents.alert.fix.css defines .alert-default — BS3-era class; dropped in BS4, not reintroduced in BS5.

All three are dead-on-arrival under BS5. The first belongs to the per-skin layer this issue tracks. The other two are default-group CSS, so strictly they fall outside the "per-skin loading" framing — but the content audit naturally covers them too: each fix file should justify its rules against the current target Bootstrap version.