Execute the configured analytics load callback safely

[jdevalk]

Summary

Fixes #38

Run the configured callback through the script's onload handler instead of the unused data-onload attribute. WordPress escapes the handler, and the browser regression verifies execution with quoted callback text.

Only users with unfiltered_html can authorize callback code. A site-specific signature records authorized saves. Previously stored callbacks remain inactive until an authorized administrator saves the Advanced settings again; changing WordPress authentication salts also requires saving again.

Resolved the requested conflicts against current main. The merged version retains the manual-pageview fix from #42, both-state event-setting tests from #43, and the existing regression harness and lifecycle fixes. Manual collection still emits data-auto-collect="false", while the callback is supplied by OnloadCallback::get().

Testing

• WordPress 7.1 / PHP 8.3.33: all 19 browser tests and all 3 WordPress regression tests passed.
• Tests cover both manual-collection states, actual callback execution, authorized and rejected saves, inactive legacy or modified callback values, clearing the handler, IP sanitization, and plugin deactivation/uninstall behavior.
• Frozen-lockfile install, asset build, PHP syntax, and diff checks passed.