fix(plugin): validate provides.mixins targets against allowlist (#2256)

PackageLoader silently accepted unknown mixin targets: a typo like
"controler" or an unsupported name like "view" produced zero mixin
injection, so the package appeared loaded but its methods never reached
the target. Added $validateMixinTargets() to reject unknown targets up
front with a clear error naming the unknown entry and the full allowlist.
Dropped the silent-noop "view" target from the basecoat and hotwire
manifests (controller mixins already surface in views via variables
scope) and aligned the CLAUDE.md provides.mixins description with the
actual allowlist.

Refs #2247

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bpamiri

package.json

@@ -5,7 +5,7 @@
 	"description": "Basecoat UI component helpers for Wheels. shadcn/ui-quality design using plain HTML + Tailwind CSS. No React required.",
 	"wheelsVersion": ">=3.0",
 	"provides": {
-		"mixins": "controller,view",
+		"mixins": "controller",
 		"services": [],
 		"middleware": []
 	},