Command handler registry: migrate projector to handler pattern

[harrryyd]

What to build

Apply the same handler registry pattern from #136 to projectEvent in apps/server/src/orchestration/projector.ts. Instead of a switch over event type, use a ProjectionHandlerRegistry where each event type registers its projection handler.

Each h-code event type (thread.archived-and-new-created, thread.trim-point-created) moves to its own file alongside its corresponding command handler (established in #136). Upstream event types migrate to their own handler files too.

Acceptance criteria

• ProjectionHandlerRegistry created (reusing MethodRegistry pattern from Pre-factor: MethodRegistry generic utility #132)
• projectEvent dispatches through registry instead of switch
• h-code event projections moved to dedicated files alongside command handlers
• All existing projector behavior preserved (tests pass)
• vp check and vp run typecheck pass

Blocked by

• Command handler registry: core + migrate decider switch #136
• RPC registry: migrate upstream methods + remove old infra #135 (for the MethodRegistry migration-complete state, though development can start once Command handler registry: core + migrate decider switch #136 is done)