fix: remove Swift 6 captured-var warnings in Server startup

[solderzzc]

Three startup vars were captured by concurrently-executing closures. Today that's a warning; in the Swift 6 language mode it's an error:

Site (main 804616e)	var	Why it's a var
Server.swift:1337	isVision	the VLM→LLM fallback (#173) flips it after a failed VLM load; read in /health
Server.swift:1056, :1368	mtpAssistantModelRef	assigned after the assistant loads; read in container.perform and in the chat route

Each is now captured through a let snapshot taken after its last mutation: loadedAsVision, loadedMTPAssistant, and a local assistant for the mainModelRef wiring. DualModelMTP is class-bound (via Module), so setting through the let still reaches the same object. No behaviour change.

Verified

• Clean rebuild (Xcode 27 / Swift 6.4): no captured-var warnings remain.
• Mac mini M6 smoke test: unsloth/Qwen3.6-35B-A3B-UD-MLX-4bit (VLM fallback) returns /health "vision":false, Qwen3.8-27B-4bit returns "vision":true, and Gemma 4 with --mtp --mtp-assistant-model …assistant-bf16 prints MTP assistant ready (3 tokens/round). All three answer a request.

Independent of #183; the hunks don't overlap.

🤖 Generated with Claude Code

[solderzzc]

Reviewed (code-review skill, M5 agent): no issues found. Both snapshots are taken after the last write: isVision is last set at the VLM fallback (line 936) and mtpAssistantModelRef at line 1050. Every later read uses the let copies, so behavior is unchanged. I'll merge once CI is green.