assets: add audio placeholder files and freesound recommendations for Sprint 19

Adds 11 empty .ogg placeholders (4 music tracks, 7 SFX) so the AudioManager
can reference them without errors during development. Includes docs/audio-assets-sprint19.md
with curated freesound.org download recommendations (CC0/CC-BY) for each file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/audio-assets-sprint19.md

@@ -0,0 +1,65 @@
+# Audio Assets — Sprint 19
+
+Download these files manually from freesound.org and replace the empty placeholders under `assets/audio/`.
+
+All picks are CC0 (Public Domain) unless noted as CC-BY. CC-BY files require attribution in the game's credits / README.
+
+---
+
+## Music tracks
+
+| Target file | Freesound ID | Title | Author | License | Duration | URL |
+|---|---|---|---|---|---|---|
+| `assets/audio/music/floor_0.ogg` | 725019 | Kids Background - Happy Children (loop ver.2) | AudioCoffee | CC0 | ~60s loop | https://freesound.org/people/AudioCoffee/sounds/725019/ |
+| `assets/audio/music/floor_0.ogg` *(alt)* | 720612 | Kids - Children Background (loop) | AudioCoffee | CC0 | ~60s loop | https://freesound.org/people/AudioCoffee/sounds/720612/ |
+| `assets/audio/music/floor_1.ogg` | 387588 | Piano Ambiance 4 (120bpm) — Ambient Piano Loop 37 | Erokia | CC0 | loop | https://freesound.org/people/Erokia/sounds/387588/ |
+| `assets/audio/music/floor_1.ogg` *(alt)* | 384934 | Soft Piano Loop #1 | ispeakwaves | CC-BY 3.0 | loop | https://freesound.org/people/ispeakwaves/sounds/384934/ |
+| `assets/audio/music/floor_2.ogg` | 684511 | Simple Game Music Loop | Seth_Makes_Sounds | CC0 | loop | https://freesound.org/people/Seth_Makes_Sounds/sounds/684511/ |
+| `assets/audio/music/floor_2.ogg` *(alt)* | 387588 | Piano Ambiance 4 — use if floor_1 already taken | Erokia | CC0 | loop | https://freesound.org/people/Erokia/sounds/387588/ |
+| `assets/audio/music/floor_3.ogg` | 723913 | Forest birds - ambient seamless loop | Magnesus | CC0 | seamless loop | https://freesound.org/people/Magnesus/sounds/723913/ |
+| `assets/audio/music/floor_3.ogg` *(alt)* | 798842 | Morning Birds in a Quiet Urban Garden | WhisperingEarth | check page | field recording | https://freesound.org/people/WhisperingEarth/sounds/798842/ |
+
+> **floor_2.ogg note:** The nursery/lullaby track is the hardest to find as a pure CC0 loop. `Seth_Makes_Sounds/684511` is a gentle, simple game loop that works for a calm nursery atmosphere. If you want a proper lullaby feel, `ispeakwaves/384934` (Soft Piano Loop, CC-BY 3.0) is a better fit — just add attribution.
+
+---
+
+## SFX
+
+| Target file | Freesound ID | Title | Author | License | Duration | URL |
+|---|---|---|---|---|---|---|
+| `assets/audio/sfx/chest_tap.ogg` | 679772 | Knocking on Wood | ominouswhoosh | CC0 | ~0.3s | https://freesound.org/people/ominouswhoosh/sounds/679772/ |
+| `assets/audio/sfx/chest_tap.ogg` *(alt)* | 617056 | Tapping Wood 1 | F.M.Audio | CC0 | short | https://freesound.org/s/617056/ |
+| `assets/audio/sfx/item_spawn.ogg` | 683096 | Woosh | florianreichelt | CC0 | short | https://freesound.org/people/florianreichelt/sounds/683096/ |
+| `assets/audio/sfx/item_spawn.ogg` *(alt)* | 460473 | VS_Short Whoosh 8 | Vilkas_Sound | CC0 | short | https://freesound.org/people/Vilkas_Sound/sounds/460473/ |
+| `assets/audio/sfx/item_drag_start.ogg` | 411177 | Pick up Item 1 | SilverIllusionist | CC0 | short | https://freesound.org/people/SilverIllusionist/sounds/411177/ |
+| `assets/audio/sfx/item_drag_start.ogg` *(alt)* | 133280 | game pick up object | Leszek_Szary | CC0 | short | https://freesound.org/people/Leszek_Szary/sounds/133280/ |
+| `assets/audio/sfx/item_drop_hand.ogg` | 448086 | Normal click | Breviceps | CC0 | <0.2s | https://freesound.org/people/Breviceps/sounds/448086/ |
+| `assets/audio/sfx/item_drop_hand.ogg` *(alt)* | 580780 | Flashlight clicking sound | StrikeWhistler | CC0 | <0.2s | https://freesound.org/people/StrikeWhistler/sounds/580780/ |
+| `assets/audio/sfx/item_drop_outfit.ogg` | 161415 | cape-swoosh | CosmicEmbers | CC-BY 3.0 | short | https://freesound.org/people/CosmicEmbers/sounds/161415/ |
+| `assets/audio/sfx/item_drop_outfit.ogg` *(alt)* | 683096 | Woosh (florianreichelt) — softer pick | florianreichelt | CC0 | short | https://freesound.org/people/florianreichelt/sounds/683096/ |
+| `assets/audio/sfx/item_return_chest.ogg` | 740266 | soft button click 1 | FOSSarts | CC0 | <0.2s | https://freesound.org/people/FOSSarts/sounds/740266/ |
+| `assets/audio/sfx/item_return_chest.ogg` *(alt)* | 677860 | UI Button Click Snap | el_boss | CC0 | <0.2s | https://freesound.org/people/el_boss/sounds/677860/ |
+| `assets/audio/sfx/item_drop_floor.ogg` | 449955 | Wooden Thud (Mono) | Breviceps | CC0 | <0.3s | https://freesound.org/people/Breviceps/sounds/449955/ |
+| `assets/audio/sfx/item_drop_floor.ogg` *(alt)* | 342530 | Light Thud 4 | sgrowe | CC0 | <0.3s | https://freesound.org/people/sgrowe/sounds/342530/ |
+
+---
+
+## Download instructions
+
+1. Open each URL in a browser while logged in to freesound.org (free account required for download).
+2. Click "Download" and choose OGG format where available (or download as WAV and convert with Audacity/ffmpeg).
+3. Rename the file to match the "Target file" column and drop it into the repo, replacing the empty placeholder.
+4. For CC-BY files, add an entry to `docs/credits-audio.md` (create if it does not exist yet) with: `Author — Title — CC-BY 3.0 — URL`.
+
+## ffmpeg conversion (WAV to OGG)
+
+```
+ffmpeg -i input.wav -c:a libvorbis -q:a 4 output.ogg
+```
+
+## Notes
+
+- All CC0 picks require no attribution, but you may credit anyway.
+- Prefer the primary pick; use the alt only if the primary is unavailable or unsuitable on preview.
+- Keep music loops between 30–60 s to minimize file size on mobile.
+- SFX should be trimmed with a short (~5 ms) fade-out to avoid clicks.