Add visual effects system documentation

docs/how/effects.rst

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+==============
+visual effects
+==============
+
+the overlay system for transient visual effects like cloud trails.
+
+overview
+========
+
+effects are temporary overlays displayed on top of terrain in the viewport.
+each effect has a position, character, color, and expiration time. they're
+rendered last-wins on overlap, cleaned up passively in the game loop.
+
+data model
+==========
+
+effect dataclass
+----------------
+
+defined in ``effects.py``::
+
+    @dataclass
+    class Effect:
+        x: int
+        y: int
+        char: str
+        color: str  # ANSI code
+        expires_at: float  # monotonic time
+
+global state
+------------
+
+single global list::
+
+    active_effects: list[Effect] = []
+
+no per-zone partitioning. simple.
+
+core functions
+==============
+
+``add_effect(x, y, char, color, ttl)``
+    creates effect with expiry at ``time.monotonic() + ttl``, appends to list
+
+``get_effects_at(x, y)``
+    returns active effects at position, auto-filters expired ones
+
+``clear_expired()``
+    in-place list mutation to drop expired effects. called once per tick
+    (10/sec) in ``server.py`` game loop
+
+rendering
+=========
+
+in ``look.py``, for each viewport tile:
+
+1. check ``get_effects_at(x, y)``
+2. if effects exist, take ``effects[-1]`` (most recent)
+3. display its ``char`` and ``color``, overlaying terrain
+
+last-added effect wins on overlap.
+
+timing
+======
+
+uses ``time.monotonic()`` for consistent async timing. ttl in seconds.
+
+current usage: fly command creates cloud trail ("~" bright white) with
+staggered ttls (1.5s base + 0.4s per step) so clouds fade origin to destination.
+
+design decisions
+================
+
+- global state, not per-zone (simplicity)
+- passive cleanup via tick-based ``clear_expired()``
+- last-wins overlay (no z-index, no blending)
+- just char + ANSI color (minimal rendering)
+- monotonic time (no wall clock drift)
+
+code
+====
+
+- ``src/mudlib/effects.py`` - dataclass, functions, global state
+- ``src/mudlib/look.py`` - viewport rendering with effect overlay
+- ``src/mudlib/server.py`` - game loop calls ``clear_expired()``
+- ``content/commands/fly.toml`` - cloud trail effect usage