jawstree: release removed Treeview instances

[linkdata]

Summary

• own each browser Treeview from its live Jid container instead of retaining per-instance window globals
• make the element Jid the sole browser identity by removing the unused Tree key and key fields from both server-to-client payloads
• resolve callbacks and server selection updates through the live DOM so detached widgets become garbage-collectible
• cover shared-Tree remove/reinsert behavior with both the focused adapter mock and the real JaWS plus vendored Treeview assets under jsdom

Tests

• go generate ./...
• go vet ./...
• gofmt -l .
• staticcheck ./...
• golangci-lint run
• gosec -quiet ./...
• JAWS_REQUIRE_NODE=1 go test -count=1 -race -coverprofile=/private/tmp/jaws-issue135-key-coverage.out ./...
• go build -v ./...
• GOOS=linux GOARCH=386 CGO_ENABLED=0 go test -c -o /private/tmp/jaws-bind-key-386.test ./lib/bind
• GOOS=linux GOARCH=386 CGO_ENABLED=0 go test -c -o /private/tmp/jaws-jawstree-key-386.test ./jawstree

Closes #135