feat(packaging): bundle ffmpeg in the Windows installer by default

winget install ffmpeg was considered and rejected as the mechanism: it
needs network access and winget/App Installer present at the exact
moment setup runs, and its failure mode is silent -- video just does
not stream, with nothing pointing back at ffmpeg. Not viable for a
non-technical install.

MeshBay transcodes browser-incompatible video to H.264 (-c:v libx264,
webrtc_server.py) -- a real encode, not remux -- so this needs a genuine
GPL ffmpeg build; no LGPL-only build includes an H.264 encoder, since
libx264 itself is GPL.

packaging/win/fetch-ffmpeg.ps1 (new)
  Downloads, checksum-verifies and stages ffmpeg for the build. Source:
  BtbN/FFmpeg-Builds' Windows x86_64 gpl-shared preset -- shared DLLs
  rather than two independent static binaries, which is what nearly
  tripled this: the "full" static build many devs already have via
  winget is ~220 MB *per executable*. Pinned to one dated release tag
  (immutable once published) and its own sha256, not the "latest" alias
  BtbN repoints on every auto-build -- verified by hand first (downloaded,
  hash matched, ran a real encode+probe with libx264) before pinning.
  ffplay.exe (an SDL2 player, ~17 MB) is dropped; MeshBay never invokes
  it. Cached after the first build. Runs its own smoke test (encode +
  probe a real clip) so a broken fetch fails at build time, not for the
  first user who tries to watch something.

packaging/win/LICENSE-ffmpeg.txt (new)
  GPLv3 notice + where the corresponding source is, required because
  this redistributes a GPL binary even though it is unmodified and only
  ever invoked as a subprocess. Ships alongside ffmpeg.exe in the
  installer.

build-node-runtime.ps1 / build-win.ps1
  Bundling is now the DEFAULT, replacing the old opt-in -FfmpegDir (which
  copied from a local directory and left most builds without ffmpeg at
  all). -SkipFfmpeg opts out for a smaller, streaming-less local-iteration
  build.

  Also fixes a real bug the ffmpeg change exposed rather than caused: the
  final `--help` smoke test did `$help -notmatch "meshbay-node"` against
  $help captured as a PowerShell ARRAY (one element per line) -- -notmatch
  on a collection is a FILTER, not a boolean test, and returns the
  non-matching elements; any non-empty array is truthy in if() regardless
  of content. Once --help wrapped past one line (it now does, with
  autostart/service in the verb list) this threw unconditionally. Fixed
  by joining to one string before matching, and pinned by a new test so
  a future edit cannot silently reintroduce the collection-vs-scalar trap.

Verified: downloaded and hashed the pinned release by hand (matches),
ran a real libx264 encode + ffprobe against the extracted build,
fetch-ffmpeg.ps1 end to end (161 MB staged), a full build-node-runtime.ps1
run (308 MB node-runtime/) and a full installer build (MeshBay-Setup-
1.0.0.exe, 210.8 MB with ffmpeg bundled). Node suite 850 pass / 25 skip.

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

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