Keep asset packs out of git and out of Godot's importer

The packs total 3.9GB, almost all of it Helton Yan's SFX shipped as 24-bit /
96kHz studio masters -- ~2MB for a single gunshot. Two guards, because they
solve different problems:

- .gitignore excludes the contents. The directory and its guard files stay
  tracked, so a fresh clone has the .gdignore in place before anyone drops
  packs back in.
- assetpacks/.gdignore stops Godot descending into the tree at all. This is the
  urgent half: tools/check.sh runs --import, which walks all of res://, so the
  next check would have imported several thousand PNGs and 2,100 of those WAVs,
  and every run after it. Verified by controlled test rather than assumed -- an
  unguarded PNG under res:// imports and gets a .import file beside it; with
  .gdignore neither happens. check.sh stays at 3.3s and .godot/imported at
  1.2MB.

The trade the .gdignore buys is that nothing in assetpacks/ can be referenced
from a scene or script. That is the intended shape: raw source on one side,
game-ready files copied into res://assets/ on the other, so adding a huge pack
costs the repo and the import step nothing.

docs/ASSETS.md records attribution, because gitignoring the packs also
gitignores their licence files. Worth flagging: four of the six packs shipped
with no licence text at all, and two of the undecided ones are demo versions,
which are often more restricted than the paid packs. Only RF Catacombs has
explicit terms (public domain, no resale). Those rows need a source URL before
anything ships.

Also removed __MACOSX and .DS_Store extraction artefacts, and left
considering_dont_use_yet/ untouched as the user's own "not chosen" marker.

.gitignore

@@ -1,5 +1,14 @@
 # Godot's import cache and editor state. Regenerated by `godot --import`.
 .godot/
+# Raw asset packs: gigabytes of source artwork and audio. Redistributable
+# third-party material belongs where it was bought or downloaded, not in a code
+# repo. The directory itself and its guards stay tracked, so a fresh clone has
+# the .gdignore in place before anyone drops packs back in -- without it the
+# next `tools/check.sh` tries to import the lot.
+/assetpacks/*
+!/assetpacks/.gdignore
+!/assetpacks/README.md
+
 # Export artefacts.
 build/
 *.pck