docs: VM handoff — CLAUDE.md, Ubuntu bootstrap, headless-Sway setup for M0

Prepares the move to the NVIDIA-GPU Ubuntu VM where M0/M2 run (macOS can't drive the
Wayland/GPU stack). The repo carries the context, since Claude Code sessions are
machine-local and don't transfer.

- CLAUDE.md: project state + design invariants + don't-regress security notes. Auto-loads
  every session, so a fresh session on the VM continues from here.
- scripts/bootstrap-ubuntu.sh: verifies the (already-installed) NVIDIA/NVENC stack,
  installs rustup + PipeWire/portal/wlroots/Sway + DRM/EGL/GBM/VA dev deps; GATES the
  FFmpeg -dev headers so apt can't clobber a custom NVENC build; checks nvidia-drm.modeset.
- scripts/headless/: headless-Sway + xdg-desktop-portal-wlr config templates, the
  NVIDIA-wlroots env workarounds, run-headless-sway.sh, and a wf-recorder->hevc_nvenc
  capture smoke test (proves capture->NVENC with no Rust).
- docs/linux-setup.md: M0 walkthrough + verified gotchas (modeset, headless backend,
  vGPU NVENC licensing, dmabuf->NVENC CPU-copy fallback, FFmpeg-dev gate, crate versions).

Ubuntu 24.04 package names/versions verified against the live archive; scripts pass
shellcheck and `bash -n`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/headless/capture-smoke-test.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Prove the capture->NVENC chain end to end WITHOUT writing any Rust, using wf-recorder
+# (wlr-screencopy) piped into hevc_nvenc. Run from a shell where headless Sway is up and
+# WAYLAND_DISPLAY + XDG_RUNTIME_DIR are exported (see run-headless-sway.sh output).
+#
+# Usage: scripts/headless/capture-smoke-test.sh [output.mkv] [WxH-unused]
+#   Ctrl-C to stop; then play/inspect the file (e.g. ffprobe out.mkv).
+set -euo pipefail
+
+OUT="${1:-/tmp/lumen-headless-test.mkv}"
+: "${WAYLAND_DISPLAY:?set WAYLAND_DISPLAY (e.g. wayland-1) — is headless Sway running?}"
+: "${XDG_RUNTIME_DIR:?set XDG_RUNTIME_DIR=/run/user/\$(id -u)}"
+
+command -v wf-recorder >/dev/null || { echo "wf-recorder missing — run scripts/bootstrap-ubuntu.sh"; exit 1; }
+
+echo "recording HEADLESS-1 -> $OUT  (wlr-screencopy -> hevc_nvenc, low-latency). Ctrl-C to stop."
+# Low-latency NVENC params verified for FFmpeg 6.x/7.x: preset p1, tune ull, no B-frames.
+exec wf-recorder -o HEADLESS-1 -c hevc_nvenc \
+    -p preset=p1 -p tune=ull -p rc=cbr -p bf=0 -p delay=0 \
+    -f "$OUT"