feat: add kernel ACPI support for querying battery percentage, add battery indicator to desktop, move log state to 0:/state directory

docs/power.md

@@ -82,6 +82,40 @@ IA32_PM_ENABLE, HWP requests, and MSR_POWER_CTL do not survive suspend.
 BSP setup, and bumps the policy epoch so every AP re-applies its HWP
 request (and re-checks C1E) on its next tick.
 
+### Battery (`ACPI/Battery.cpp`, `ACPI/EmbeddedController.cpp`)
+
+Battery state comes from the ACPI Control Method Battery
+(`_HID PNP0C0A`) and, for the AC adapter, `ACPI0003`:
+
+- `_STA` bit 4 says whether a pack is installed. A laptop with the pack
+  removed still has the device, which is why "has a battery device" and
+  "has a battery" are separate answers.
+- `_BIX` (or `_BIF` on older firmware) gives the power unit (mWh vs mAh),
+  the design capacity, and the last full charge capacity.
+- `_BST` gives the charge/discharge state, the present rate, the remaining
+  capacity and the present voltage.
+- `_PSR` on the adapter says whether it is plugged in.
+
+Percentage is remaining / last-full, clamped to 100 and falling back to the
+design capacity when firmware reports last-full as zero. Results are cached
+for two seconds so that a panel repaint does not turn into EC traffic.
+
+Almost all of this reaches the CPU through the **embedded controller**.
+`Hal::EmbeddedController` finds the EC's command/data port pair from the
+ECDT table when one exists, and otherwise from `_CRS` on the `PNP0C09`
+device, then drives the polled read/write protocol (`0x80`/`0x81` with
+OBF/IBF handshaking). It is wired into the AML interpreter as the
+`EmbeddedControl` region space, so any `OperationRegion` in the DSDT that
+targets the EC now works -- battery methods being the first user.
+
+Supporting `_BST` also needed real package element storage in the AML
+interpreter. Firmware overwhelmingly builds its result package with
+`Store(x, Index(BST0, n))`, which requires `Index()` to name a writable
+element rather than yield a value. `AML::PackageStore` provides that: a
+persistent region for packages declared with `Name()`, and a transient ring
+rewound at each top-level evaluation for packages built inside a method, so
+polling `_BST` cannot exhaust it.
+
 ## Diagnostics
 
 `SYS_POWERINFO` (149) fills a `PowerInfo` struct: package temperature,
@@ -90,7 +124,15 @@ CPUID base/turbo frequency, the measured average active frequency
 (APERF/MPERF over the 500 ms governor window), and the AP idle MWAIT hint.
 Returns -1 when the CPU exposes no power features (e.g. under QEMU).
 
-The `power` program (`programs/src/power`) prints it:
+`SYS_BATTERYINFO` (188) fills a `BatteryInfo` struct: percentage, charge
+state, AC presence, capacities, present rate, voltage and an estimated time
+remaining. It returns -1 on a machine with no battery device at all, which
+is how the desktop panel decides not to draw an indicator. Like
+`SYS_POWERINFO` it is ungated read-only telemetry: the panel indicator has
+to work in an unprivileged session, which holds no device capability to
+spend on it.
+
+The `power` program (`programs/src/power`) prints both:
 
 ```
 power             one-shot snapshot
@@ -104,8 +146,17 @@ Boot log lines to look for (Kernel Log app):
 [CpuPower] HWP enabled: perf range 4-54, EPP=0x80
 [CpuPower] Thermal governor armed: TjMax=100C, passive=85C, hot=94C, ...
 [CpuIdle]  AP deep idle: MWAIT hint 0x20
+[EC]       Embedded Controller at cmd=66 data=62
+[Battery]  ACPI battery at \_SB.PCI0.LPCB.EC0.BAT0 (AC adapter present)
 ```
 
+The desktop panel shows a battery icon left of the Wi-Fi and network icons,
+using the icon pack's `battery-level-N[-charging]-symbolic` glyphs (one per
+10% step, reloaded only when the step changes). Clicking it opens a popout
+with the percentage in numbers, the charge state, time remaining and raw
+capacity. The icon is absent entirely when `SYS_BATTERYINFO` fails, so a
+desktop machine shows nothing there.
+
 ## Tuning
 
 The governor constants live at the top of `Hal/CpuPower.cpp`