ffmpeg: fdk-aac is opt-in (from-source build) — consider a build switch

[ns408]

Context

We use the homebrew-ffmpeg/ffmpeg tap (PR #12) with the default bottled build: libass, drawtext/libfreetype, and many extra filters, but not fdk-aac.

Enabling fdk-aac requires brew "homebrew-ffmpeg/ffmpeg/ffmpeg", args: ["with-fdk-aac"], which:

• forces a from-source compile (no bottle, since FDK AAC is non-free), and
• recompiles ffmpeg on every version bump — heavy for the unattended daily-update LaunchDaemon.

Decision (current)

Stay on the bottled build; ffmpeg's native AAC encoder is fine for current needs. The trade-off is noted inline in packages/Brewfile.developer and packages/Brewfile.cloud-engineer.

Future enhancement: a build switch

Let the user opt into the FDK AAC build without editing the Brewfile. Options to weigh:

1. Brewfile conditional (Ruby). Brewfiles are Ruby, so:

   brew "homebrew-ffmpeg/ffmpeg/ffmpeg", args: ENV["NS_FFMPEG_FDK_AAC"] ? ["with-fdk-aac"] : []

   Set NS_FFMPEG_FDK_AAC=1 before brew bundle to get the source build.

2. bootstrap flag / machine config. A --ffmpeg-fdk-aac flag (or a key in ~/.config/ns-bootstrap/config) that the install step reads to decide the args.

Either way, document the source-build + recompile-on-update cost, and consider pinning/skipping ffmpeg in the unattended updater when the source build is selected so a 3am rebuild doesn't stall.

Acceptance

• A documented, single switch to choose bottled vs with-fdk-aac.
• Default stays bottled.
• Unattended updater not silently doing heavy source rebuilds.