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.

docs/ASSETS.md

@@ -0,0 +1,54 @@
+# Asset packs and attribution
+
+The packs themselves are **not in git** (see
+[`assetpacks/README.md`](../assetpacks/README.md)), which means their bundled
+licence files are not either. This file is the record that survives. Keep it
+current when a pack is added or removed.
+
+## Why nothing is imported yet
+
+`assetpacks/` carries a `.gdignore`, so Godot skips it entirely. Nothing there
+can be referenced from a scene or script until it is copied into
+`res://assets/` in game-ready form. That is deliberate: the tree is 3.9 GB, and
+`tools/check.sh` runs `--import`, which would otherwise walk all of it on every
+run.
+
+## Packs
+
+| Pack | Size | Intended use | Licence |
+| --- | --- | --- | --- |
+| 0x72 Dungeon Tileset II v1.7 | 1.6 MB | Dungeon tiles — maps to the `MapGrid.Kind` tiles the generator already emits | **Unrecorded.** Bundled README is technical, not legal. Widely distributed as CC0 by Robert Norenberg (0x72), but confirm at the source before shipping. |
+| Effect and FX Pixel All Free | 29 MB | Hit flashes, muzzle flashes, death effects | **Unrecorded** — no licence file in the pack. |
+| Helton Yan's Pixel Combat | 3.9 GB | Combat SFX | **Unrecorded** — no licence file in the pack. |
+| New All Fire Bullet Pixel 16x16 | 360 KB | Bullet sprites, to replace the generated dot in `bullet_renderer.gd` | **Unrecorded** — no licence file in the pack. |
+| `considering_dont_use_yet/` — RF Catacombs v1.0 | part of 1.6 MB | Undecided | **Public domain**, per bundled `public-license.txt`. "Free to use, personal or commercial. Credit not required but appreciated. You can edit, but not resell the asset pack." Artwork by Szadi art. |
+| `considering_dont_use_yet/` — Tiny RPG Character Pack 02, neo_zero v1.0 / 3.0 demo | part of 1.6 MB | Undecided | **Unrecorded.** |
+
+### Action needed before shipping anything
+
+Four of the six packs arrived with no licence text at all. Redistribution terms
+for pixel-art and audio packs vary a lot — some free packs forbid commercial
+use, some require credit, and "free demo" versions (two of the undecided packs
+are demos) are often more restricted than the paid ones.
+
+Record the source URL and terms for each **Unrecorded** row above before any of
+it goes into a build. The `considering_dont_use_yet/` packs are the user's own
+"not chosen yet" marker; leave them there until that changes.
+
+## Converting audio before use
+
+Helton Yan's pack is 24-bit / 96 kHz stereo — studio masters, averaging ~2 MB
+per file and 5 MB at the largest. That is roughly a hundred times what a game
+needs for a gunshot.
+
+Downsample to 16-bit / 44.1 kHz mono before copying anything into
+`res://assets/`. Godot imports `.wav` losslessly and keeps it in memory
+uncompressed for low-latency playback, so the source format is what you pay for
+in RAM and in export size.
+
+## Where converted assets go
+
+`res://assets/`, which is committed and imported. See
+[`assets/README.md`](../assets/README.md). Art is Stage 2 in
+[ROADMAP.md](ROADMAP.md); everything currently on screen is drawn from
+primitives plus one runtime-generated dot texture.

docs/ROADMAP.md

@@ -4,6 +4,9 @@
 brief to its current state and the files that implement it, so you can find the
 relevant code without re-reading the whole project.
 
+Art and audio source packs, and their licence status, are recorded in
+[ASSETS.md](ASSETS.md) — the packs themselves are not in git.
+
 Design decisions that are already settled — and the reasoning behind them — live
 in [DECISIONS.md](DECISIONS.md). Check there before asking the user something
 that may already have an answer.