Add time/season/weather system documentation

docs/how/time-and-weather.rst

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+=============================
+time and weather system
+=============================
+
+the game simulates passing time, seasons, weather, and their effect on visibility. all together these create atmospheric descriptions in the look command and influence how far you can see.
+
+game time
+=========
+
+time flows faster in the game world than in reality. by default, 1 real minute = 1 game hour, so a full game day passes in 24 real minutes.
+
+the ``GameTime`` class converts real timestamps to game time using an epoch (when the game started) and a ratio. methods::
+
+    get_game_hour() -> int       # 0-23
+    get_game_time() -> tuple     # (hour, minute)
+    get_game_day() -> int        # days since epoch, 0-based
+
+``init_game_time()`` is called at server startup to establish the global game clock. all other systems query this clock to determine current conditions.
+
+time of day
+===========
+
+game hours map to periods: dawn (5-6), day (7-17), dusk (18-19), night (20-4).
+
+``get_sky_description(hour)`` picks variant descriptions for each period. the selection is deterministic using ``hour % len(variants)`` so the same hour always shows the same description within that day. examples::
+
+    dawn:  "pale light seeps across the horizon"
+    day:   "the sun hangs high overhead"
+    dusk:  "the day's light begins to soften"
+    night: "stars wheel slowly overhead"
+
+there are 3-4 variants per period to add variety across different hours.
+
+seasons
+=======
+
+the game year is 28 days long with 4 seasons of 7 days each: spring, summer, autumn, winter. seasons cycle infinitely.
+
+``get_season(game_day)`` returns the current season name. ``get_season_description(season, terrain)`` returns descriptive text, but only for grass and forest terrain. other terrain types return empty strings. examples::
+
+    spring + grass:  "fresh green grass springs up everywhere"
+    winter + forest: "bare branches reach toward the sky"
+    summer + forest: "a thick green canopy spreads overhead"
+
+this is layered into the atmosphere line when looking at the world.
+
+weather
+=======
+
+weather is a global simulation that evolves each game hour.
+
+conditions
+----------
+
+``WeatherCondition`` enum: clear, cloudy, rain, storm, snow, fog.
+
+``WeatherState`` tracks the current condition plus intensity (0.0-1.0). intensity affects descriptions::
+
+    rain < 0.3:  "a light drizzle falls"
+    rain 0.3-0.6: "rain patters steadily"
+    rain >= 0.6: "rain hammers down relentlessly"
+
+similar tiers exist for snow, fog, and storm.
+
+climate profiles
+----------------
+
+``advance_weather()`` is probabilistic with three climate profiles:
+
+- **temperate**: balanced transitions between all conditions
+- **arid**: 90% chance clear stays clear, rare rain, no snow
+- **arctic**: heavy emphasis on snow and fog
+
+season filtering prevents snow in spring/summer.
+
+global state
+------------
+
+``init_weather()`` sets up the initial condition. ``get_current_weather()`` returns the current state. ``tick_weather()`` is called each game hour to advance the simulation.
+
+visibility
+==========
+
+weather and time of day affect how far you can see.
+
+``get_visibility(hour, weather, base_width=21, base_height=11)`` calculates effective viewport dimensions by applying stacking penalties::
+
+    night:           -6 width, -2 height
+    dawn/dusk:       -2 width
+    thick fog (>=0.7): -8 width, -4 height
+    fog (0.4-0.7):   -4 width, -2 height
+    storm:           -4 width, -2 height
+
+minimum viewport is 7x5. effects stack: a stormy night with thick fog applies all three penalties.
+
+integration
+===========
+
+the look command pulls current hour, day, weather, and season. it calculates effective viewport via ``get_visibility()`` and builds the atmosphere line with ``render_atmosphere()``.
+
+example outputs::
+
+    "the sun hangs high overhead. [day, summer]"
+
+    "the day's light begins to soften. rain patters steadily. [day, autumn]"
+
+    "stars wheel slowly overhead. snow drifts down softly. [night, winter]"
+
+the atmosphere line appears at the top of the world view, followed by the terrain map clipped to effective visibility.
+
+code
+====
+
+relevant files::
+
+    src/mudlib/gametime.py
+    src/mudlib/timeofday.py
+    src/mudlib/seasons.py
+    src/mudlib/weather.py
+    src/mudlib/visibility.py
+    src/mudlib/commands/look.py