spike(embedded-mpv): auto-detect Homebrew prefix and Node headers for Intel Macs

BREW_PREFIX was hardcoded to /opt/homebrew (Apple Silicon) and NODE_INC to
one nvm version; both now resolve via brew --prefix and the PATH node's
execPath, so the pending Intel Mac run needs no Makefile edits. README gets
a fresh-machine checklist.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

spikes/mpv-frame-copy/Makefile

@@ -1,11 +1,12 @@
 # mpv frame-copy spike (macOS only for now).
 #
-# Requires: Homebrew libmpv (brew install mpv) and Node headers for the
-# N-API addon. Override NODE_INC if your Node lives elsewhere:
-#   make NODE_INC=$(node -e 'console.log(process.execPath)')/../../include/node
+# Requires: Homebrew libmpv (brew install mpv) and Node >= 18 on PATH (for
+# its N-API headers). Both are auto-detected; override BREW_PREFIX/NODE_INC
+# if detection picks the wrong one. Works on Apple Silicon (/opt/homebrew)
+# and Intel (/usr/local) Homebrew layouts.
 
-BREW_PREFIX ?= /opt/homebrew
-NODE_INC ?= $(HOME)/.nvm/versions/node/v22.14.0/include/node
+BREW_PREFIX ?= $(shell brew --prefix 2>/dev/null || echo /opt/homebrew)
+NODE_INC ?= $(shell node -p "require('path').join(process.execPath, '..', '..', 'include', 'node')" 2>/dev/null)
 
 BUILD := build
 

spikes/mpv-frame-copy/README.md

@@ -20,8 +20,20 @@ to measure whether the copy pipeline meets the go/no-go gates.
 
 ## Build and run
 
-Requires Homebrew `mpv` (libmpv + headers) and Node headers for the addon
-build (`NODE_INC` in the Makefile, defaults to nvm's v22.14.0).
+Requires Homebrew `mpv` (libmpv + headers) and Node ≥ 18 on `PATH`; the
+Makefile auto-detects the Homebrew prefix (Apple Silicon and Intel) and the
+Node include dir.
+
+Quick checklist for a fresh machine (e.g. the pending Intel Mac run):
+
+```bash
+brew install mpv                     # libmpv + headers
+git clone <repo> && cd <repo>
+pnpm install --frozen-lockfile       # provides the Electron binary for the viewer
+cd spikes/mpv-frame-copy
+./run.sh 'av://lavfi:testsrc2=size=1920x1080:rate=60'   # sanity
+# then the RESULTS.md scenarios (4K synthetic, 4K HEVC hwdec, 50 fps, HDR)
+```
 
 ```bash
 ./run.sh /path/to/video.mkv 3840x2160