Expose read-only device power info to mods

docs/rfcs/0008-device-power-info.md

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+# RFC 0008 — Read-only device power information for sandboxed mods
+
+## Status
+
+Proposed. Engine: `Loader.lua`, `Sandbox.lua`. Test:
+`tests/modkit/cases/device_power_info.lua`.
+
+## Motivation
+
+A handheld UI mod can show the player's battery state and warn before power
+loss. The sandbox correctly removes `love.system` because that module also
+launches URLs and exposes other host operations, but it leaves no scoped way
+to read the harmless power values that LÖVE already provides.
+
+## The decision it extends
+
+Extends the mod sandbox in `src/mods/Sandbox.lua`: blocked host modules stay
+blocked while legitimate operations receive narrow engine-owned facades.
+
+## The exact API delta
+
+Add `mod.device:powerInfo() -> state, percent`.
+
+The engine calls `love.system.getPowerInfo()` outside the mod sandbox and
+returns only its first two values. `state` is one of LÖVE's standard power
+states. `percent` is `0` through `100` or `nil`. When the platform has no
+power-information backend, the result is `"unknown", nil`.
+
+No permission grants access to `love.system`; URL launching, clipboard access,
+OS identification, and the module table itself remain unavailable.
+
+## Migration note for existing mods
+
+Mods that used `love.system.getPowerInfo()` replace that call with
+`mod.device:powerInfo()`. No other mod changes.
+
+## Parity tests
+
+- **No mod:** loading no mods does not call the platform power backend.
+- **Mod API:** a fixture mod loaded through the public loader receives state
+  and percentage through `mod.device`, while the existing sandbox suite keeps
+  proving that direct `love.system` access is refused.
+- **Unavailable backend:** the public facade returns `"unknown", nil` rather
+  than inventing battery data or failing mod load.
+
+## Deprecation etiquette
+
+Nothing deprecated. The facade is additive; the sandbox's `love.system` block
+remains in force.