fix: log JsVar rejections and handler panics without alerts

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JsVar proposal rejections and recovered event-handler panics produce generic browser alerts that users cannot act on. Log these failures without automatic alerts, preserving JsVar rollback, canonical corrections, and size-limit cancellation. Applications can send useful feedback explicitly with source.Request.Alert(...) inside ClientCheck. Ordinary returned handler errors still produce danger alerts.

Replace the internal clientAlertError text-selection interface and helper with ErrEventLogOnly, matched through errors.Is in browser-event and test-hook dispatch. Recovered panics and JsVar rejection errors match this sentinel; explicit alerts remain unaffected. Update the callback and runtime documentation.

Validation:

• Regression tests cover logged details, returned errors, panics, hooks, wrapped log-only errors, explicit alerts, JsVar rollback/correction, read-only stores, ErrEventUnhandled, and size-limit cancellation.
• JAWS_REQUIRE_NODE=1 go test -race ./... and JAWS_REQUIRE_NODE=1 go test ./... pass.
• go generate ./..., go vet ./..., staticcheck ./..., golangci-lint run, gosec -quiet ./..., and go build ./... pass.
• gofmt, changed-file gofumpt, git diff --check, and rendered go doc review pass.