docs(backlog): record PPD-vs-TLP default power backend as a Decision

Bernardo asked whether TLP would be more power-efficient and worth making
the default. Logged in § Decisions with the evidence gathered 2026-07-08:
TLP's gain is idle-only and unquantified (its own maintainer won't cite
numbers), the sibling distro Omarchy tried TLP-for-battery and reverted to
PPD, and TLP-default would rework the powerprofilesctl-based profile UX.
Agent recommendation: keep PPD, chase battery via targeted low-risk tweaks
+ keep TLP the one-line opt-in. Decision itself stays with Bernardo.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/BACKLOG.md

@@ -327,3 +327,29 @@ decide.
   Firefox is installed and are skipped otherwise, so the remaining call
   is only: ship a browser active in the suite, or stay
   browserless-by-default?
+
+- **Default power backend — PPD vs TLP:** (raised by Bernardo 2026-07-08:
+  would TLP be more power-efficient, and should it be the default?)
+  TLP does get more *idle* battery life, but only from device-level knobs
+  PPD deliberately omits — PCIe ASPM, disk/NVMe link PM, USB autosuspend,
+  runtime PM — which are the same knobs behind NVMe/USB flakiness that
+  pillar 1 (rock-stable, never fight your machine) guards against.
+  **Evidence (2026-07-08):** no credible hard-watt benchmarks exist — TLP's
+  own maintainer declines to quote any ("measure on your hardware") and says
+  PPD's power-saver gives *similar* savings under load; TLP's edge is
+  idle-only (linrunner.de/tlp/faq/ppd.html — PPD covers a *subset* of TLP,
+  "no settings to reduce consumption when the CPU is idle"). Our sibling
+  distro **Omarchy** ran this exact experiment — a "replace Power Profiles
+  with TLP for battery" guide — and the author **reverted to PPD** ("more
+  headaches and weird issues"); Omarchy shipped PPD auto-switching instead
+  (basecamp/omarchy#3907). PPD 3.4.0 now does AC/battery auto-switching,
+  closing part of TLP's UX gap. **Cost of TLP-default:** TLP has no live
+  D-Bus profile API, so the `powermgmt` menu + Waybar profile indicator +
+  low-battery auto power-saver (all `powerprofilesctl`) would need a rework —
+  it's not a one-line backend swap. **Agent rec (evidence, not a decision):
+  keep PPD default;** chase battery via targeted low-risk tweaks (PCIe ASPM
+  policy, battery-side EPP, the charge *start* threshold, PPD auto-switching)
+  and keep TLP the documented one-line opt-in it already is
+  (`nomarchy.system.power.backend = "tlp"`). Options: (i) status quo +
+  targeted tweaks [rec]; (ii) TLP default (reworks the profile UX);
+  (iii) nothing.