`pyssg.plugins.api`

Plugin API surface.

A plugin is a composition root: it only declares facts (units, connections, aspects, cache versions). The engine owns every incremental algorithm, so as long as a plugin's declarations are honest (the purity contract) it cannot break incremental correctness.

This module is peripheral (not pyssg.core), so it MAY import third-party libs -- but the base protocol here needs none.

`class Plugin(Protocol)`

Structural contract every plugin satisfies.

cache_version folds into the cache key of every unit the plugin creates; bump it whenever the plugin's behavior changes so stale cached output is busted.

`Plugin.apply(self, builder: Builder) -> None`

`class PluginContext`

Convenience handle passed to plugin helpers.

Thin in M2; gains declaration helpers (define_aspect etc.) as later milestones wire the aspect/collection machinery.