fix: never discover a plugin's dependency, build or asset directories

[ogorzalka]

Problem

A plugin with neither app/ nor src/ was discovered from its root. That is the shape of a plugin that only ships blocks — blocks are registered by convention and need no class — and such a plugin has node_modules/ at its root as soon as it has a Vite build.

Measured on pollora-test (pollora-blocks-demo, blocks in resources/views/blocks, no app/): every request logged Pollora discovery scanned …/pollora-blocks-demo in 2168–6985ms, front page at 3–8 s.

Change

PluginRegistrar::getPluginDiscoveryPaths() (was getPluginDiscoveryPath()):

• app/, else src/ — unchanged;
• otherwise the plugin's other top-level directories, never node_modules, vendor, bower_components, build, dist, public, resources, assets, languages, lang or a hidden directory. A plugin with classes in e.g. includes/ is still discovered; a blocks-only plugin is not scanned at all.

Tests

• PluginDiscoveryPathTest: other top-level directories scanned; none of the excluded ones ever returned.
• On pollora-test, same plugin without app/: front page 0.55–0.68 s, no slow-scan warning, pollora-blocks-demo/accordion still registered.
• Full suite serial 1368 green. (The one parallel-only failure is the flaky slow-scan test fixed in test: warm up discovery before timing an empty scan #371.)