examples/pong: production-quality Pong game with powerups and multi-ball

A complete, playable Pong game split across examples/pong/*.ne files
and pulled in from a top-level examples/pong.ne. Features:

- **Title screen** with a 3-option menu (CPU VS CPU / 1 PLAYER /
  2 PLAYERS), a cursor sprite, blinking "PRESS A" prompt, brisk
  title march on pulse 2, and autopilot that auto-confirms CPU VS
  CPU after 45 frames of no input so the headless jsnes golden
  harness reaches gameplay by frame 180.

- **Ball physics** with signed-magnitude velocity (u8 magnitude +
  sign bit per axis), wall bounce at top/bottom, paddle AABB
  collision with push-out, and score-out detection at left/right
  exits.

- **Multi-ball** via parallel ball_* arrays (MAX_BALLS = 3). Each
  ball scores a point independently; the round continues until the
  last ball exits the playfield.

- **CPU AI** that tracks the nearest active ball heading toward its
  side with a per-frame step, 4 px dead zone, and CPU_SPEED = 1 so
  rallies can end naturally.

- **Three powerup types** that spawn every ~4 seconds, bounce off
  all four walls, and are caught by paddle AABB overlap:
  1. LONG — extends the catching paddle from 24 → 40 px for 5 hits
  2. FAST — doubles ball x-velocity on the catcher's next hit
  3. MULTI — spawns two extra balls on the catcher's next hit

- **Victory** at first-to-7 with a "PLAYER N WINS" banner and the
  builtin fanfare, auto-returning to Title.

- **Audio**: 5 user-declared sfx (WallBounce, PaddleHit, Score,
  PowerSpawn, PowerCatch) plus a title march and the builtin
  fanfare for victory.

Source layout mirrors examples/war:

    examples/pong.ne               top-level game shell
    examples/pong/PLAN.md          living design doc
    examples/pong/constants.ne     layout + gameplay constants
    examples/pong/assets.ne        45-tile Tileset (paddles, ball, alphabet,
                                   digits, cursor, center-line, powerup icons)
    examples/pong/audio.ne         sfx + music declarations
    examples/pong/state.ne         all mutable globals
    examples/pong/rng.ne           8-bit Galois LFSR
    examples/pong/render.ne        draw helpers
    examples/pong/input.ne         paddle step (human + CPU AI)
    examples/pong/ball.ne          multi-ball physics + paddle collision
    examples/pong/powerup.ne       powerup entity (spawn, bounce, catch, apply)
    examples/pong/title_state.ne   state Title + menu
    examples/pong/play_state.ne    state Playing (P_SERVE/P_PLAY/P_POINT)
    examples/pong/victory_state.ne state Victory

Verification:
- 616 compiler unit tests pass (cargo test --all-targets)
- cargo fmt / cargo clippy --all-targets -- -D warnings clean
- 33/33 emulator harness goldens match
- examples/pong.nes builds byte-identically from source

https://claude.ai/code/session_0134F5uwDEVTes2Ee9S7JeXy

tests/emulator/goldens/pong.audio.hash

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