Add Seasonal effect and animate the holiday effects

Seasonal picks the holiday effect from the date, using an NTP clock set to
the follow_me_timezone substitution (default UTC). Dates: New Year's, Valentine's,
St. Patrick's, Easter (Good Friday to Easter Monday), Haunted and Halloween,
Hanukkah (computed from the Hebrew calendar), Christmas and Autumn. Other
days show the plain glow. The date logic was checked against independent
Hebrew-calendar and Easter libraries for 2020 to 2060.

Autumn gets falling, swaying, tumbling leaves with periodic gusts and a brown
leaf color. Candy Cane gets a gloss glint, Easter a shimmer and sparkles,
Valentine's rising hearts, New Year's fireworks, and Halloween a bat flyby
and lightning.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

firmware/follow-me.yaml

@@ -102,6 +102,11 @@
 #     themselves do nothing in ESPHome; the stream paints them. Turning the
 #     light off resets the effect to None (ESPHome behaviour). At boot, an
 #     effect name without a pattern is logged as an error.
+#   - Seasonal: an effect that shows the holiday theme for today's date
+#     (follow-me-effects.h, seasonal_effect) and the plain glow on other
+#     days. The date comes from an NTP clock set to the substitution
+#     follow_me_timezone; until the clock is set, Seasonal shows the plain
+#     glow.
 #   - Night light: while the "Follow-Me Night Light" light is on, a patch
 #     "Follow-Me Night Light Size" steps long is lit at each end of the
 #     stairs, with a raised-cosine fade over NL_EDGE_STEPS on its inner side.
@@ -141,6 +146,14 @@ substitutions:
   # target number, 1-3. The defaults are Apollo's.
   radar_x_name: "Target-%d X"
   radar_y_name: "Target-%d Y"
+  # Time zone for the Seasonal effect's dates (a tz database name such as
+  # America/New_York).
+  follow_me_timezone: UTC
+
+time:
+  - platform: sntp
+    id: follow_me_time
+    timezone: ${follow_me_timezone}
 
 globals:
   # Follow-Me Glow channel values, 0-1.
@@ -244,6 +257,10 @@ light:
     # Effects only name a pattern; the stream paints it (follow-me-effects.h,
     # whose EFFECT_NAMES must match these names).
     effects:
+      - lambda:
+          name: "Seasonal"
+          update_interval: 1h
+          lambda: "// Painted by the follow-me stream."
       - lambda:
           name: "Rainbow"
           update_interval: 1h
@@ -907,7 +924,11 @@ interval:
           // Follow-Me Glow effect: replaces the colour with a per-LED pattern
           // at the light's brightness (transitions included).
           const StringRef fx_name = id(follow_me_glow).get_effect_name();
-          const follow_me::fx::Effect effect = follow_me::fx::from_name(fx_name.c_str(), fx_name.size());
+          follow_me::fx::Effect effect = follow_me::fx::from_name(fx_name.c_str(), fx_name.size());
+          if (effect == follow_me::fx::SEASONAL) {
+            const ESPTime clock = id(follow_me_time).now();
+            effect = follow_me::fx::resolve(effect, clock.is_valid(), clock.year, clock.month, clock.day_of_month);
+          }
           const LightColorValues &glow_now = id(follow_me_glow).current_values;
           const float fx_peak = glow_now.get_state() * glow_now.get_brightness() * 255.0f;
           const float fx_t = (now % 86400000u) / 1000.0f;  // wraps daily so float seconds keep ms resolution