examples/platformer: polish pass (art, audio, more levels, gamepad)

[markwpearce]

Follow-up polish work for examples/platformer ("Jungle Adventure"), tracked as one issue since these are all smaller, independent items rather than a single feature.

• New sound effects - currently reuses generic/reused sounds from other examples (jump.wav/stomp.wav from pong's hit.wav, coin.wav from the shared score.wav, die.wav from the scaffold template) - get dedicated sounds for jump, coin pickup, enemy stomp, and player death.
• Proper enemy sprite - Enemy currently renders as a flat red DrawableRectangle placeholder, no sprite/animation.
• Make text more readable - a first sizing pass already landed (font sizes bumped, layout re-checked against title-safe margins), but the win panel wasn't independently verified on-device and a couple of gaps were called out as tight - worth a fresh look with actual gameplay/art in place, not just isolated screenshots.
• Enable gamepad - analog-stick/gamepad input support already exists at the BGE.Controller.ControlMap level (bound alongside the Roku remote d-pad), but hasn't had real hands-on verification with an actual connected gamepad/controller, only the remote.
• Add more levels - currently one level; the original design scope (issue Add an examples/platformer (side-scrolling, gravity + jumping) #62) anticipated "maybe two." Needs a simple level-index/progression mechanism (Goal advances to the next level instead of triggering the win screen until the last one) plus increasing difficulty (more enemies, longer gaps, tighter one-way-platform timing).
• Add soundtrack/music - no background music currently.
• Get "goal" artwork - Goal currently renders as a flat blue DrawableRectangle placeholder, no dedicated sprite.

Stretch goal:

• Vine swinging mechanic - a new movement ability, bigger scope than the rest of this list, evaluate separately once the core polish above is done.

See specs/2026-09-05-platformer-example-design.md / specs/2026-09-05-platformer-example-plan.md for the original build's design/plan context.

[markwpearce]

Also update credits to include stuff about the sprites and the backgrounds:

Backgrounds - https://opengameart.org/content/sunnyland-tall-forest-environment
Character sprites: https://opengameart.org/content/adventurer-girl-free-sprite

[markwpearce]

investigate why walking left on simulator does not flip the character sprite

[markwpearce]

While investigating issue #211 with the debug collider overlay enabled on examples/platformer (real device), noticed the coin colliders visually don't line up with their sprites - the collider outline is offset from the drawn coin circle (seen in MainRoom, coin spawns from LevelData.bs's o tiles).

Not investigated further yet since it's unrelated to #211 - flagging here since it looked like it might belong under this issue's umbrella.