Remove non-redistributable art from the repo; record licence terms

Researched the licence at each of the four itch.io sources. Two of the four
forbid redistribution, and publishing a repository containing them is exactly
that, so they are out of the tree.

  0x72 DungeonTileset II    CC0 1.0    commercial ok, no credit required
  Helton Yan Pixel Combat   CC BY 4.0  commercial ok, CREDIT REQUIRED
  Fire Pixel Bullet 16x16   custom     non-commercial, NO REDISTRIBUTION
  750+ Effect and FX Pixel  custom     non-commercial, NO REDISTRIBUTION

Both bdragon1727 packs state verbatim: "You cannot do: Resell / redistribute
this asset." The FX pack's paid tier is no better for this purpose -- "You can
NOT re-distribute the file, no matter how much you modify it" -- so paying does
not unlock it and neither does editing the art.

bullets.png and impact.png therefore move to assets/local/, gitignored.
Art.bullets_texture() / impact_texture() load them at runtime and return null
when absent; the bullet renderer falls back to the generated dot it used before,
tinted per kind so bullet types stay distinguishable. Nothing there may be
preload()ed -- preload resolves at compile time and would fail the build on
every machine without the files, which is every clone.

Verified both ways rather than assumed: with the files present the game uses
them and no fallback triggers; with them moved aside, 156/156 tests pass and the
game runs clean on the generated dots.

Added CREDITS.md, because Helton Yan's CC BY 4.0 makes attribution a licence
obligation rather than a courtesy. A file in the source tree does not discharge
it once the game ships -- an in-game credits screen is now a tracked todo, and
flagged as a licence requirement rather than polish.

NOT addressed here, and it needs a decision: these files remain in git history
at 9fa2e26. Removing them from the tip does not remove them from a clone. If
this repo is ever made public, history has to be rewritten (git filter-repo)
and force-pushed. Also, both bdragon packs are non-commercial in their free
tier, which conflicts with the stated Steam goal -- either contribute for the
commercial tier (still no redistribution) or replace with CC0 art, which would
solve both problems and let the assets back into the repo.

CREDITS.md

@@ -0,0 +1,30 @@
+# Credits
+
+Third-party assets used in Transcience, with the attribution their licences
+require.
+
+## Audio
+
+**Pixel Combat SFX** — [Helton Yan](https://heltonyan.itch.io/pixelcombat)
+Licensed under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).
+Converted to 16-bit / 44.1 kHz mono and trimmed for use in game.
+
+> Attribution is **required** by this licence, not optional. It has to appear
+> somewhere a player can reach — this file is not enough on its own once the
+> game ships. See docs/ROADMAP.md.
+
+## Art
+
+**16x16 DungeonTileset II** — [0x72 (Robert Norenberg)](https://0x72.itch.io/dungeontileset-ii)
+Released under [CC0 1.0](https://creativecommons.org/publicdomain/zero/1.0/)
+(public domain). Credit is not required; it is here because the work deserves
+it.
+
+## Not distributed with this repository
+
+Two packs are used locally but deliberately **excluded** from the repository,
+because their licences forbid redistribution. They are not part of any build
+made from this source alone. See [docs/ASSETS.md](docs/ASSETS.md).
+
+- Fire Pixel Bullet 16x16 — [bdragon1727](https://bdragon1727.itch.io/fire-pixel-bullet-16x16)
+- 750+ Effect and FX Pixel All — [bdragon1727](https://bdragon1727.itch.io/750-effect-and-fx-pixel-all)