fix(server): v0.5.1 follow-ups — GUI TTL wiring + in-flight-safe sweep, kwargs to TUI/MCP, alias-keyed kwargs

[magicnight]

Finishes the v0.5.1 follow-ups flagged in the #49 review.

A4 — per-model idle TTL:

• Wired into the GUI: EngineCoordinator.load reads ModelParameters.ttlSeconds → ModelPool.load(ttlSeconds:).
• Fixed the sweepIdle mid-use hazard: PooledEngineEntry.isGenerating in-flight marker; EngineCoordinator.generate marks the entry around the stream (cleared on success/throw/cancel via defer); sweepIdle skips in-flight entries.

A5 — template kwargs:

• Wired to CLI ChatTUI (interactive) and MCPBridge (both previously dropped kwargs).
• Server templateKwargs(for:) resolves an alias to the real model id before loading kwargs.

Tests: testSweepIdleSkipsInFlightGeneratingEntry, aliasResolvesToModelWithTemplateKwargs. 173 tests pass; MacMLXCore + CLI + GUI all build clean (independently verified).

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