docs: reposition as the 86 Panel development environment

The repo's documentation framed it as a support repo for one product
(WardenOS). Since going public the real audience is anyone with a Luckfox
Pico 86 Panel: a maintained 6.18 kernel, an off-device development loop,
and a device simulator that exist nowhere else for this board. Reframe the
README and top-level docs board-first, with WardenOS documented as the
downstream consumer it is (ADR-0008).

Also an editorial pass over the whole doc set:
- every H1/H2 is now a short title, not a sentence (ADRs, qemu/, patches/,
  drivers/, architecture, NPU feasibility, config-lint, payload); workflow
  flowchart titles fixed at the source in tools/flowgen.py and regenerated
  with fresh bench numbers
- README Quick Start commands verified against the scripts; requirements
  corrected (curl, bare python, gcc >= 14) and the MC/DC gate added as a
  step (run green locally on gcc 14.2)
- dropped the 'needs python (not python3)' vendor dig: build-kernel.sh
  inherited the same requirement (filed #10 to remove it)
- glossed MC/DC and HPMCU on first use; marked the tests/uboot-ab
  reference as flare-edge; deduplicated the three-simulator list into the
  root README table

qemu/payload/README.md

@@ -1,6 +1,7 @@
-# qemu/payload/ — guest binaries (never committed)
+# Guest Payloads
 
-Drop **static musl armv7** binaries here; `qemu/mkimage.sh` copies everything
+Drop **static musl armv7** binaries in this directory (contents are
+gitignored — binaries are never committed); `qemu/mkimage.sh` copies everything
 in this directory (except this README) into `/usr/bin/` of both rootfs slots.
 Static musl is the same target the device uses for its Rust daemons, so the
 exact production binaries run unmodified in the VM.