[SDK-4] Conflict resolution runtime — throw-by-default

Implement conflict resolution from [RFC #5](https://github.com/ChrisonSimtian/Fallout/issues/101).

## Scope

- Detect conflicting registrations (duplicate `IHost` for same env, duplicate middleware `(phase, priority)`, duplicate tool wrapper name, etc.).
- Default behaviour: throw with structured error naming both plugins and the conflicting contribution.
- Override mechanism per RFC's final form.
- No silent winner-takes-all.

## Depends on

SDK-3 and the per-surface extension-point tickets (SDK-6a..6f).