feat: cel-shaded look & feel overhaul — real audio, comic UI, themed VFX

Audio (all CC0 — Kenney packs + OpenGameArt, see assets/sounds/SOURCES.md):
- Replace every procedural synth WAV with forged sounds (ffmpeg pitch/layer
  mixes): distinct fire sounds per weapon with 2-3 variations, explosions,
  footsteps x5, land/jump/dash/vault, knife swing, hit/kill confirms,
  bullet impacts, reload, flight loops, UI hover/click/confirm/error,
  match jingles, menu music
- AudioManager.stream_for(): AudioStreamRandomizer per sound id — every
  weapon now gets variation + pitch randomization on each shot
- Explosions and bullet impacts play positional audio; landing has its own
  sound instead of a pitched footstep; ambient wind bed on every map

Visuals:
- ExplosionVFX: shared cel-shaded burst (white-hot stepped core, ink
  shockwave ring, star spikes, flat smoke puffs) replaces the orange
  sphere in both local and remote-replay paths
- Comic star muzzle flashes on all weapons; unified cel tracer bolts with
  ink outlines across hitscan/shotgun/remote paths
- Impact decals: hard-stepped ink-splat gradients instead of soft airbrush
- Toon shading on first-person view models + arms, third-person weapons,
  and the procedural humanoid fallback (no hull outlines on FBX weapons —
  their hard normals tear the inverted hull)
- Fix: giant soft "blob" highlight on floors — toon rim/specular disabled
  on level-geometry materials (pre-existing artifact since the cel commit)
- Fix: own third-person weapon rendered into the first-person camera
  (shadows-only now covers first_person_mode)

UI:
- UITheme: comic theme on the root window — Bangers display font (OFL),
  paper panels with thick ink borders + hard drop shadows, papaya accent,
  themed buttons/inputs/popups; hover/click sounds on all menu buttons
- Main menu: tilted comic wordmark, sunset toon diorama, looping menu music
- Match HUD: themed scoreboard/killfeed/kill counter

Viewports:
- 4x MSAA project-wide + viewmodel/diorama viewports (crisp ink lines)
- Viewmodel camera near plane 0.01; third-person camera FOV syncs settings
- debug/visual_capture.gd: dev tool to screenshot menu + level for review

Verified: 11/11 movement tests, spawn smoke test 0 failures, before/after
screenshot comparison of menu and level.

Co-Authored-By: Claude Fable 5 <[email protected]>

assets/sounds/SOURCES.md

@@ -0,0 +1,16 @@
+# Sound sources
+
+All current WAVs are forged (pitch/layer/trim via ffmpeg — see repo history)
+from these CC0 packs, no attribution required:
+
+- Kenney Impact Sounds — https://kenney.nl/assets/impact-sounds (CC0)
+- Kenney Sci-Fi Sounds — https://kenney.nl/assets/sci-fi-sounds (CC0)
+- Kenney Interface Sounds — https://kenney.nl/assets/interface-sounds (CC0)
+- Kenney Music Jingles — https://kenney.nl/assets/music-jingles (CC0)
+
+Exceptions (still procedural from `generate_sounds.py`): `wind.wav`,
+`wallrun.wav`, `slide.wav` — seamless noise loops that the Kenney one-shots
+can't replace.
+
+Menu music: `music_menu.wav` — "Menu Music" by mrpoly, CC0,
+https://opengameart.org/content/menu-music

assets/ui/fonts/OFL-Bangers.txt

@@ -0,0 +1,93 @@
+Copyright 2010 The Bangers Project Authors (https://github.com/googlefonts/bangers)
+
+This Font Software is licensed under the SIL Open Font License, Version 1.1.
+This license is copied below, and is also available with a FAQ at:
+https://scripts.sil.org/OFL
+
+
+-----------------------------------------------------------
+SIL OPEN FONT LICENSE Version 1.1 - 26 February 2007
+-----------------------------------------------------------
+
+PREAMBLE
+The goals of the Open Font License (OFL) are to stimulate worldwide
+development of collaborative font projects, to support the font creation
+efforts of academic and linguistic communities, and to provide a free and
+open framework in which fonts may be shared and improved in partnership
+with others.
+
+The OFL allows the licensed fonts to be used, studied, modified and
+redistributed freely as long as they are not sold by themselves. The
+fonts, including any derivative works, can be bundled, embedded, 
+redistributed and/or sold with any software provided that any reserved
+names are not used by derivative works. The fonts and derivatives,
+however, cannot be released under any other type of license. The
+requirement for fonts to remain under this license does not apply
+to any document created using the fonts or their derivatives.
+
+DEFINITIONS
+"Font Software" refers to the set of files released by the Copyright
+Holder(s) under this license and clearly marked as such. This may
+include source files, build scripts and documentation.
+
+"Reserved Font Name" refers to any names specified as such after the
+copyright statement(s).
+
+"Original Version" refers to the collection of Font Software components as
+distributed by the Copyright Holder(s).
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+Original Version, by changing formats or by porting the Font Software to a
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+writer or other person who contributed to the Font Software.
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+in Original or Modified Versions, may be sold by itself.
+
+2) Original or Modified Versions of the Font Software may be bundled,
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