feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/toml_edit-0.22.27/src/de/table_enum.rs

@@ -0,0 +1,176 @@
+use crate::de::Error;
+
+/// Deserializes table values into enum variants.
+pub(crate) struct TableEnumDeserializer {
+    value: crate::Item,
+}
+
+impl TableEnumDeserializer {
+    pub(crate) fn new(value: crate::Item) -> Self {
+        TableEnumDeserializer { value }
+    }
+}
+
+impl<'de> serde::de::VariantAccess<'de> for TableEnumDeserializer {
+    type Error = Error;
+
+    fn unit_variant(self) -> Result<(), Self::Error> {
+        match self.value {
+            crate::Item::ArrayOfTables(values) => {
+                if values.is_empty() {
+                    Ok(())
+                } else {
+                    Err(Error::custom("expected empty array", values.span()))
+                }
+            }
+            crate::Item::Value(crate::Value::Array(values)) => {
+                if values.is_empty() {
+                    Ok(())
+                } else {
+                    Err(Error::custom("expected empty table", values.span()))
+                }
+            }
+            crate::Item::Table(values) => {
+                if values.is_empty() {
+                    Ok(())
+                } else {
+                    Err(Error::custom("expected empty table", values.span()))
+                }
+            }
+            crate::Item::Value(crate::Value::InlineTable(values)) => {
+                if values.is_empty() {
+                    Ok(())
+                } else {
+                    Err(Error::custom("expected empty table", values.span()))
+                }
+            }
+            e => Err(Error::custom(
+                format!("expected table, found {}", e.type_name()),
+                e.span(),
+            )),
+        }
+    }
+
+    fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, Self::Error>
+    where
+        T: serde::de::DeserializeSeed<'de>,
+    {
+        seed.deserialize(super::ValueDeserializer::new(self.value))
+    }
+
+    fn tuple_variant<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: serde::de::Visitor<'de>,
+    {
+        match self.value {
+            crate::Item::ArrayOfTables(values) => {
+                let values_span = values.span();
+                let tuple_values = values.values.into_iter().collect::<Vec<_>>();
+
+                if tuple_values.len() == len {
+                    serde::de::Deserializer::deserialize_seq(
+                        super::ArrayDeserializer::new(tuple_values, values_span),
+                        visitor,
+                    )
+                } else {
+                    Err(Error::custom(
+                        format!("expected tuple with length {len}"),
+                        values_span,
+                    ))
+                }
+            }
+            crate::Item::Value(crate::Value::Array(values)) => {
+                let values_span = values.span();
+                let tuple_values = values.values.into_iter().collect::<Vec<_>>();
+
+                if tuple_values.len() == len {
+                    serde::de::Deserializer::deserialize_seq(
+                        super::ArrayDeserializer::new(tuple_values, values_span),
+                        visitor,
+                    )
+                } else {
+                    Err(Error::custom(
+                        format!("expected tuple with length {len}"),
+                        values_span,
+                    ))
+                }
+            }
+            crate::Item::Table(values) => {
+                let values_span = values.span();
+                let tuple_values: Result<Vec<_>, _> = values
+                    .items
+                    .into_iter()
+                    .enumerate()
+                    .map(|(index, (key, value))| match key.get().parse::<usize>() {
+                        Ok(key_index) if key_index == index => Ok(value),
+                        Ok(_) | Err(_) => Err(Error::custom(
+                            format!("expected table key `{}`, but was `{}`", index, key.get()),
+                            key.span(),
+                        )),
+                    })
+                    .collect();
+                let tuple_values = tuple_values?;
+
+                if tuple_values.len() == len {
+                    serde::de::Deserializer::deserialize_seq(
+                        super::ArrayDeserializer::new(tuple_values, values_span),
+                        visitor,
+                    )
+                } else {
+                    Err(Error::custom(
+                        format!("expected tuple with length {len}"),
+                        values_span,
+                    ))
+                }
+            }
+            crate::Item::Value(crate::Value::InlineTable(values)) => {
+                let values_span = values.span();
+                let tuple_values: Result<Vec<_>, _> = values
+                    .items
+                    .into_iter()
+                    .enumerate()
+                    .map(|(index, (key, value))| match key.get().parse::<usize>() {
+                        Ok(key_index) if key_index == index => Ok(value),
+                        Ok(_) | Err(_) => Err(Error::custom(
+                            format!("expected table key `{}`, but was `{}`", index, key.get()),
+                            key.span(),
+                        )),
+                    })
+                    .collect();
+                let tuple_values = tuple_values?;
+
+                if tuple_values.len() == len {
+                    serde::de::Deserializer::deserialize_seq(
+                        super::ArrayDeserializer::new(tuple_values, values_span),
+                        visitor,
+                    )
+                } else {
+                    Err(Error::custom(
+                        format!("expected tuple with length {len}"),
+                        values_span,
+                    ))
+                }
+            }
+            e => Err(Error::custom(
+                format!("expected table, found {}", e.type_name()),
+                e.span(),
+            )),
+        }
+    }
+
+    fn struct_variant<V>(
+        self,
+        fields: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: serde::de::Visitor<'de>,
+    {
+        serde::de::Deserializer::deserialize_struct(
+            super::ValueDeserializer::new(self.value).with_struct_key_validation(),
+            "", // TODO: this should be the variant name
+            fields,
+            visitor,
+        )
+    }
+}