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/hashbrown/src/control/group/lsx.rs

@@ -0,0 +1,128 @@
+use super::super::{BitMask, Tag};
+use core::mem;
+use core::num::NonZeroU16;
+
+use core::arch::loongarch64::*;
+
+pub(crate) type BitMaskWord = u16;
+pub(crate) type NonZeroBitMaskWord = NonZeroU16;
+pub(crate) const BITMASK_STRIDE: usize = 1;
+pub(crate) const BITMASK_MASK: BitMaskWord = 0xffff;
+pub(crate) const BITMASK_ITER_MASK: BitMaskWord = !0;
+
+/// Abstraction over a group of control tags which can be scanned in
+/// parallel.
+///
+/// This implementation uses a 128-bit LSX value.
+#[derive(Copy, Clone)]
+pub(crate) struct Group(m128i);
+
+// FIXME: https://github.com/rust-lang/rust-clippy/issues/3859
+#[allow(clippy::use_self)]
+impl Group {
+    /// Number of bytes in the group.
+    pub(crate) const WIDTH: usize = mem::size_of::<Self>();
+
+    /// Returns a full group of empty tags, suitable for use as the initial
+    /// value for an empty hash table.
+    ///
+    /// This is guaranteed to be aligned to the group size.
+    #[inline]
+    #[allow(clippy::items_after_statements)]
+    pub(crate) const fn static_empty() -> &'static [Tag; Group::WIDTH] {
+        #[repr(C)]
+        struct AlignedTags {
+            _align: [Group; 0],
+            tags: [Tag; Group::WIDTH],
+        }
+        const ALIGNED_TAGS: AlignedTags = AlignedTags {
+            _align: [],
+            tags: [Tag::EMPTY; Group::WIDTH],
+        };
+        &ALIGNED_TAGS.tags
+    }
+
+    /// Loads a group of tags starting at the given address.
+    #[inline]
+    #[allow(clippy::cast_ptr_alignment)] // unaligned load
+    pub(crate) unsafe fn load(ptr: *const Tag) -> Self {
+        Group(lsx_vld::<0>(ptr.cast()))
+    }
+
+    /// Loads a group of tags starting at the given address, which must be
+    /// aligned to `mem::align_of::<Group>()`.
+    #[inline]
+    #[allow(clippy::cast_ptr_alignment)]
+    pub(crate) unsafe fn load_aligned(ptr: *const Tag) -> Self {
+        debug_assert_eq!(ptr.align_offset(mem::align_of::<Self>()), 0);
+        Group(lsx_vld::<0>(ptr.cast()))
+    }
+
+    /// Stores the group of tags to the given address, which must be
+    /// aligned to `mem::align_of::<Group>()`.
+    #[inline]
+    #[allow(clippy::cast_ptr_alignment)]
+    pub(crate) unsafe fn store_aligned(self, ptr: *mut Tag) {
+        debug_assert_eq!(ptr.align_offset(mem::align_of::<Self>()), 0);
+        lsx_vst::<0>(self.0, ptr.cast());
+    }
+
+    /// Returns a `BitMask` indicating all tags in the group which have
+    /// the given value.
+    #[inline]
+    pub(crate) fn match_tag(self, tag: Tag) -> BitMask {
+        unsafe {
+            let cmp = lsx_vseq_b(self.0, lsx_vreplgr2vr_b(tag.0 as i32));
+            BitMask(lsx_vpickve2gr_hu::<0>(lsx_vmskltz_b(cmp)) as u16)
+        }
+    }
+
+    /// Returns a `BitMask` indicating all tags in the group which are
+    /// `EMPTY`.
+    #[inline]
+    pub(crate) fn match_empty(self) -> BitMask {
+        unsafe {
+            let cmp = lsx_vseqi_b::<{ Tag::EMPTY.0 as i8 as i32 }>(self.0);
+            BitMask(lsx_vpickve2gr_hu::<0>(lsx_vmskltz_b(cmp)) as u16)
+        }
+    }
+
+    /// Returns a `BitMask` indicating all tags in the group which are
+    /// `EMPTY` or `DELETED`.
+    #[inline]
+    pub(crate) fn match_empty_or_deleted(self) -> BitMask {
+        unsafe {
+            // A tag is EMPTY or DELETED iff the high bit is set
+            BitMask(lsx_vpickve2gr_hu::<0>(lsx_vmskltz_b(self.0)) as u16)
+        }
+    }
+
+    /// Returns a `BitMask` indicating all tags in the group which are full.
+    #[inline]
+    pub(crate) fn match_full(&self) -> BitMask {
+        unsafe {
+            // A tag is EMPTY or DELETED iff the high bit is set
+            BitMask(lsx_vpickve2gr_hu::<0>(lsx_vmskgez_b(self.0)) as u16)
+        }
+    }
+
+    /// Performs the following transformation on all tags in the group:
+    /// - `EMPTY => EMPTY`
+    /// - `DELETED => EMPTY`
+    /// - `FULL => DELETED`
+    #[inline]
+    pub(crate) fn convert_special_to_empty_and_full_to_deleted(self) -> Self {
+        // Map high_bit = 1 (EMPTY or DELETED) to 1111_1111
+        // and high_bit = 0 (FULL) to 1000_0000
+        //
+        // Here's this logic expanded to concrete values:
+        //   let special = 0 > tag = 1111_1111 (true) or 0000_0000 (false)
+        //   1111_1111 | 1000_0000 = 1111_1111
+        //   0000_0000 | 1000_0000 = 1000_0000
+        unsafe {
+            let zero = lsx_vreplgr2vr_b(0);
+            let special = lsx_vslt_b(self.0, zero);
+            Group(lsx_vor_v(special, lsx_vreplgr2vr_b(Tag::DELETED.0 as i32)))
+        }
+    }
+}