fix(plugin): replace PluginProxy with generated interceptor classes

[markshust]

Summary

• Fixes TypeError when a plugin targets an interface (e.g., HasherInterface) and the proxy is injected into a constructor type-hinting that interface — the core bug reported by a user
• Fixes silent plugin miss when interfaces are resolved via PreferenceRegistry instead of closure bindings — plugins registered against the interface were never found
• Replaces PluginProxy (generic __call() wrapper that didn't satisfy PHP's type system) with eval-generated interceptor classes that properly implement target interfaces or extend target concrete classes
• Adds loud errors for readonly class targets and ambiguous multi-interface plugin registration
• Removes stale demo/ directory and its references from linter config

Architecture

Dual-strategy interceptor generation (no files on disk):

Scenario	Strategy
Plugin targets an interface	Generate class that implements the interface, delegates to wrapped instance
Plugin targets non-readonly concrete class	Generate class that extends the class, overrides plugged methods
Plugin targets readonly concrete class	Throw PluginException::cannotInterceptReadonly() with helpful suggestion
Multiple interfaces with plugins on same class	Throw PluginException::ambiguousInterfacePlugins()

New files:

• PluginInterceptedInterface — marker interface with getPluginTarget() (replaces instanceof PluginProxy checks)
• PluginInterception trait — core before→target→after chain logic (deliberate exception to "no traits" rule; documented rationale)
• InterceptorClassGenerator — generates interceptor classes via eval(), cached per target

Key changes:

• PluginInterceptor::createProxy(originalId, resolvedId, target) — new signature passes both the requested interface and resolved concrete class
• Container::resolve() — passes $originalId at both createProxy call sites
• PluginRegistry::getEffectiveTargetClass() — interface-aware plugin lookup
• Router.php — updated to use instanceof PluginInterceptedInterface

Test plan

• 5061 tests passing (0 failures), including 50+ new tests for the interceptor system
• Unit tests for PluginInterception trait (16 tests), InterceptorClassGenerator (20 tests), PluginInterceptor (23 tests), PluginRegistry additions (8 tests), PluginException new methods (6 tests)
• Feature integration tests covering the full stack end-to-end (12 tests)
• Meta-test verifying PluginProxy has been fully removed from the codebase
• Verified in markotalk-local with the exact bug scenario from the report — controller type-hinting HasherInterface with a #[Plugin(target: HasherInterface::class)] before plugin: no TypeError, plugin fires correctly

🤖 Generated with Claude Code