[SDK-3] Plugin discovery & load pipeline

[ChrisonSimtian]

Implement runtime discovery + load model from RFC #4.

Scope

• Assembly enumeration at startup, read [FalloutPlugin] attributes.
• Broken-plugin handling: fail-fast with structured error, never silent skip.
• Trust posture: full-trust, in-process (no AssemblyLoadContext isolation in v1).
• Load order determinism per RFC.

Depends on

SDK-2.