Stop TEG from allocating every atmos tick

[Kronyxxx]

About the PR

While looking into whether the TEG could still cause lag, I found the center was allocating fresh GasMixtures every atmos tick, including on the no-flow path.

This swaps that over to reusable per-generator transfer buffers instead, so the TEG keeps doing the same math without constantly feeding the GC.

Why / Balance

No gameplay or balance change intended here.

This is just a small internal cleanup to reduce steady server-side allocation churn from active TEGs.

Technical details

• added two reusable transfer buffers to the TEG generator component
• changed the circulator transfer helper to write into those buffers instead of returning new mixtures
• kept the existing heat transfer / power generation behavior the same

How to test

• build Content.Server
• spawn and run a TEG normally
• confirm it still generates power as expected
• confirm there are no behavior changes in normal operation
• optional: profile allocations around TEG atmos updates and verify the per-tick GasMixture churn is gone

Media

N/A

Breaking changes

None

Changelog

No changelog. Internal perf cleanup only.