Performance overlay: thermal state field

[bahacan16]

Problem

A common report is "it starts at 40-45 FPS and sinks to 20 after a few minutes". That can be the game, the runtime, or iOS lowering clocks because the phone is hot, and the in-game overlay cannot tell them apart. The existing [device-load] line has the thermal state, but it is opt-in (env.MADEIRA_DEVICE_STATS = 1) and only in the log.

Change

• In-game menu › Performance overlay gets a fifth field, Thermal (off unless chosen, saved with the game like the other fields): Cool, Warm, Hot or Critical from ProcessInfo.thermalState (nominal, fair, serious, critical). iOS throttles from "serious" (Hot) on.
• While the performance overlay is shown, a change of state is logged once: [thermal] fair -> serious at 31 FPS.
• DeviceLoadDiagnostics.thermalName is factored out of the [device-load] line and shared.

Evidence

From the fork's overlay (iPhone 17 Pro Max, iOS 27), where the same readout has been on screen since late September:

• Ghost of Tsushima, "40-45 then 20 FPS" report: the thermal state was already WARM at the start of the session, which kept the analysis from blaming the D3D12 runtime alone; the GPU time per frame rose with the game's particle count, a separate cause.
• Sessions with async submission enabled showed no clear win while the phone was WARM from the start, so measurements are now repeated on a cool phone.

Notes / risks

• App only (Library.swift, plus the overlay fields in docs/LIBRARY.md). No new setting, no new Swift file.
• The developer interface's FPS overlay is unchanged.

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🤖 Generated with Claude Code

Claude-Session: https://claude.ai/code/session_0189oLHghpaYKLk4f786a6bc