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/layers.py

@@ -16,6 +16,7 @@ from engine.effects import (
     firehose_line,
     glitch_bar,
     noise,
+    vis_offset,
     vis_trunc,
 )
 from engine.render import big_wrap, lr_gradient, lr_gradient_opposite
@@ -94,16 +95,18 @@ def render_message_overlay(
 def render_ticker_zone(
     active: list,
     scroll_cam: int,
-    ticker_h: int,
-    w: int,
-    noise_cache: dict,
-    grad_offset: float,
+    camera_x: int = 0,
+    ticker_h: int = 0,
+    w: int = 80,
+    noise_cache: dict | None = None,
+    grad_offset: float = 0.0,
 ) -> tuple[list[str], dict]:
     """Render the ticker scroll zone.
 
     Args:
         active: list of (content_rows, color, canvas_y, meta_idx)
         scroll_cam: camera position (viewport top)
+        camera_x: horizontal camera offset
         ticker_h: height of ticker zone
         w: terminal width
         noise_cache: dict of cy -> noise string
@@ -112,6 +115,8 @@ def render_ticker_zone(
     Returns:
         (list of ANSI strings, updated noise_cache)
     """
+    if noise_cache is None:
+        noise_cache = {}
     buf = []
     top_zone = max(1, int(ticker_h * 0.25))
     bot_zone = max(1, int(ticker_h * 0.10))
@@ -137,7 +142,7 @@ def render_ticker_zone(
                     colored = lr_gradient([raw], grad_offset)[0]
                 else:
                     colored = raw
-                ln = vis_trunc(colored, w)
+                ln = vis_trunc(vis_offset(colored, camera_x), w)
                 if row_fade < 1.0:
                     ln = fade_line(ln, row_fade)
 
@@ -228,11 +233,12 @@ def process_effects(
     h: int,
     scroll_cam: int,
     ticker_h: int,
-    mic_excess: float,
-    grad_offset: float,
-    frame_number: int,
-    has_message: bool,
-    items: list,
+    camera_x: int = 0,
+    mic_excess: float = 0.0,
+    grad_offset: float = 0.0,
+    frame_number: int = 0,
+    has_message: bool = False,
+    items: list | None = None,
 ) -> list[str]:
     """Process buffer through effect chain."""
     if _effect_chain is None:
@@ -242,12 +248,13 @@ def process_effects(
         terminal_width=w,
         terminal_height=h,
         scroll_cam=scroll_cam,
+        camera_x=camera_x,
         ticker_height=ticker_h,
         mic_excess=mic_excess,
         grad_offset=grad_offset,
         frame_number=frame_number,
         has_message=has_message,
-        items=items,
+        items=items or [],
     )
     return _effect_chain.process(buf, ctx)