feat(core): add Camera abstraction for viewport scrolling

- Add Camera class with modes: vertical, horizontal, omni, floating
- Refactor scroll.py and demo to use Camera abstraction
- Add vis_offset for horizontal scrolling support
- Add camera_x to EffectContext for effects
- Add pygame window resize handling
- Add HUD effect plugin for demo mode
- Add --demo flag to run demo mode
- Add tests for Camera and vis_offset

engine/scroll.py

@@ -7,6 +7,7 @@ import random
 import time
 
 from engine import config
+from engine.camera import Camera
 from engine.display import (
     Display,
     TerminalDisplay,
@@ -27,10 +28,19 @@ from engine.viewport import th, tw
 USE_EFFECT_CHAIN = True
 
 
-def stream(items, ntfy_poller, mic_monitor, display: Display | None = None):
+def stream(
+    items,
+    ntfy_poller,
+    mic_monitor,
+    display: Display | None = None,
+    camera: Camera | None = None,
+):
     """Main render loop with four layers: message, ticker, scroll motion, firehose."""
     if display is None:
         display = TerminalDisplay()
+    if camera is None:
+        camera = Camera.vertical()
+
     random.shuffle(items)
     pool = list(items)
     seen = set()
@@ -46,7 +56,6 @@ def stream(items, ntfy_poller, mic_monitor, display: Display | None = None):
     scroll_step_interval = calculate_scroll_step(config.SCROLL_DUR, ticker_view_h)
 
     active = []
-    scroll_cam = 0
     ticker_next_y = ticker_view_h
     noise_cache = {}
     scroll_motion_accum = 0.0
@@ -72,10 +81,10 @@ def stream(items, ntfy_poller, mic_monitor, display: Display | None = None):
         scroll_motion_accum += config.FRAME_DT
         while scroll_motion_accum >= scroll_step_interval:
             scroll_motion_accum -= scroll_step_interval
-            scroll_cam += 1
+            camera.update(config.FRAME_DT)
 
             while (
-                ticker_next_y < scroll_cam + ticker_view_h + 10
+                ticker_next_y < camera.y + ticker_view_h + 10
                 and queued < config.HEADLINE_LIMIT
             ):
                 from engine.effects import next_headline
@@ -88,17 +97,17 @@ def stream(items, ntfy_poller, mic_monitor, display: Display | None = None):
                 queued += 1
 
             active = [
-                (c, hc, by, mi) for c, hc, by, mi in active if by + len(c) > scroll_cam
+                (c, hc, by, mi) for c, hc, by, mi in active if by + len(c) > camera.y
             ]
             for k in list(noise_cache):
-                if k < scroll_cam:
+                if k < camera.y:
                     del noise_cache[k]
 
         grad_offset = (time.monotonic() * config.GRAD_SPEED) % 1.0
         ticker_buf_start = len(buf)
 
         ticker_buf, noise_cache = render_ticker_zone(
-            active, scroll_cam, ticker_h, w, noise_cache, grad_offset
+            active, camera.y, camera.x, ticker_h, w, noise_cache, grad_offset
         )
         buf.extend(ticker_buf)
 
@@ -110,8 +119,9 @@ def stream(items, ntfy_poller, mic_monitor, display: Display | None = None):
                 buf,
                 w,
                 h,
-                scroll_cam,
+                camera.y,
                 ticker_h,
+                camera.x,
                 mic_excess,
                 grad_offset,
                 frame_number,