docs: hardware support guide + product backlog

Add docs/HARDWARE.md (layers, install path, firmware/fingerprint/NVIDIA
deep dives, unsupported machines, contributing a DMI line). Link from
README and MIGRATION; queue firmware UI, doctor checks, detect-hw CLI,
and related PROPOSED items in BACKLOG.

agent/BACKLOG.md

@@ -517,6 +517,89 @@ flagged separately from fidelity bugs.*
   design) — optional README note so users know what ships and how to
   override. Cost: docs only.
 
+### Hardware product (investigation 2026-07-09)
+
+*Full write-up: [`docs/HARDWARE.md`](../docs/HARDWARE.md). Install-time
+autodetect is strong; day-2 lifecycle (firmware, biometrics, re-detect,
+NVIDIA depth) is still mostly CLI. Items below are product work on top
+of that doc — design notes live there (§4–§10).*
+
+- **System › Firmware (fwupd product surface)**
+  `services.fwupd.enable` is already default-on and never auto-flashes,
+  but users only learn about `fwupdmgr` from a README sentence. Add a
+  self-gated menu entry (System submenu) and/or Control Center row that:
+  refreshes metadata, lists updatable devices, confirms, runs
+  `fwupdmgr update`, then prints a reboot note when needed. Never
+  silent-flash (pillar 1). Optional: first-boot / MOTD one-liner when
+  the daemon is active. Spec: docs/HARDWARE.md §4. Cost: menu script +
+  optional MOTD; V2 menu, V3 on real LVFS hardware.
+
+- **Doctor hardware section (read-only)**
+  Extend `nomarchy-doctor` with the same pass/fail + one-fix-command
+  contract for: NetworkManager connectivity, default PipeWire sink,
+  optional GPU smoke (`vainfo`/`glxinfo` if present), fprintd unit when
+  fingerprint was enabled, fwupd active (+ warn if `get-updates`
+  non-empty), laptop charge-threshold sysfs when a limit is set. Spec:
+  docs/HARDWARE.md §10. Cost: doctor.sh sections; V1 in existing
+  `checks.doctor` where fakes exist, rest V2/V3.
+
+- **`nomarchy-detect-hw` CLI (post-install re-probe)**
+  The installer already has pure `nomarchy_detect_hw` stdout protocol
+  (MODULE / NOMARCHY / DETAIL). Ship it as a package on PATH that
+  prints suggested `hardwareProfile = [ … ]` and `system.nix` snippets
+  without reinstalling — closes the template/migration gap and “I
+  swapped the Wi‑Fi card.” Does not rewrite the flake unless a later
+  `--apply` is explicitly designed. Spec: docs/HARDWARE.md §8. Cost:
+  extract script + package + man/README pointer; V0–V1.
+
+- **Fingerprint menu: enroll / list (+ optional PAM toggle)**
+  Installer enables fprintd on known USB VIDs but enroll is CLI-only
+  (`fprintd-enroll`) and PAM is a commented rebuild. Self-gated System
+  row when `fprintd` is present: enroll, list, delete; optional “use
+  for login” that writes `settings`/system intent + rebuild note
+  (Control Center Bluetooth pattern). Spec: docs/HARDWARE.md §5.
+  Cost: menu module; V3 enroll on real reader (`[blocked:hw]` for
+  full path).
+
+- **Installer: NVIDIA commented system.nix guidance**
+  Detection already adds `common-gpu-nvidia`; there is no
+  `nomarchy.hardware.nvidia.*` and hybrid/PRIME is wiki territory.
+  When NVIDIA is detected, emit a **commented** block in generated
+  `system.nix` (same style as ROCm/NPU) pointing at PRIME /
+  powerManagement / open-module and docs/HARDWARE.md §6 — not a full
+  second stack. Cost: install script strings; V0 + doc.
+
+- **CI: hardware-db module names ∈ nixos-hardware**
+  Every `HARDWARE_DB` third field must exist in the pinned
+  `nixos-hardware.nixosModules` or installs break only on matching
+  DMI. Add a pure `checks.*` (or option-docs-style runCommand). Pairs
+  with existing “installer pure contracts” proposal. Cost: small
+  check; V0.
+
+- **Installer: `usbutils` on PATH for fingerprint probe**
+  Fingerprint path prefers `lsusb` but package wrap has `pciutils`
+  only; sysfs VID walk is the fallback. Add `usbutils` to
+  `nomarchy-install` PATH for reliable detection. Cost: one package
+  in wrap; V0.
+
+- **Document + README table: `nomarchy.hardware.i2c.*`**
+  `i2c.ddcci` is **mkDefault true** (external monitor brightness) but
+  missing from README option tables → `checks.option-docs` red (also
+  under docs-drift pack). Document in README + HARDWARE.md already
+  mentions ddcci; close option-docs. Cost: docs; V0 with option-docs.
+
+- **Post-install hardware hints (MOTD / first session)**
+  When relevant services exist, surface one line each: firmware
+  (`fwupdmgr get-updates`), fingerprint (`fprintd-enroll`), optional
+  “run nomarchy-doctor”. Avoid nagging — once or until dismissed.
+  Spec: HARDWARE.md §4/§5. Cost: small greeter/MOTD or notify; V2/V3.
+
+- **NVIDIA first-class options (larger, optional)**
+  Only if a maintainer commits to hybrid testing: thin
+  `nomarchy.hardware.nvidia.*` wrapping common PRIME/power patterns.
+  Prefer the commented-guidance item first; promote only with
+  hardware-queue coverage. Cost: high; `[big]` + `[blocked:hw]`.
+
 
 ## Decisions `[human]`