feat(remote-shell): PTY-over-mesh terminal with retro-CRT UI

Adds a full RemoteShell (portnum=13) implementation matching Jonathan's
dmshell_client.py protocol:

Protocol layer:
- Seq/ack reliability: incrementing seq on every non-ACK frame, piggybacked
  ack_seq, out-of-order frame buffering, gap detection and replay requests
- TX history ring buffer (last 50 frames) for retransmission on request
- ACK frames carry optional 4-byte big-endian REPLAY_REQUEST payload
- PING/PONG heartbeat with 8-byte status payload (lastTxSeq, lastRxSeq);
  PONG handler triggers replay if peer is behind
- PKI: DataPacket.PKC_CHANNEL_INDEX so CommandSenderImpl applies
  Curve25519 encryption (firmware rejects non-PKI DMShell packets)
- Input batching: 500ms debounce (matches Python client), immediate flush
  on \r, \t, buffer-full (64 bytes), or Enter

Terminal UI:
- Retro-CRT composables: TerminalCanvas (phosphor glow, two-pass bloom),
  ScanlinesOverlay, FlickerEffect (animated brightness variation),
  CrtCurvatureModifier (AGSL barrel distortion on Android 12+, no-op on JVM)
- PhosphorPreset enum: GREEN (P1), AMBER (P3), WHITE (P4)
- Pending-input rendered inline in preset.dim colour; snaps to confirmed on flush
- Hidden zero-size BasicTextField captures soft and hardware keyboard input
- Phosphor colour picker dropdown in top bar

Capabilities gate:
- supportsRemoteShell gated to UNRELEASED (9.9.9)
- Entry only visible in AdministrationSection when node.capabilities.supportsRemoteShell

core/data/src/commonMain/kotlin/org/meshtastic/core/data/manager/MeshDataHandlerImpl.kt

@@ -48,6 +48,7 @@ import org.meshtastic.core.repository.PacketHandler
 import org.meshtastic.core.repository.PacketRepository
 import org.meshtastic.core.repository.PlatformAnalytics
 import org.meshtastic.core.repository.RadioConfigRepository
+import org.meshtastic.core.repository.RemoteShellHandler
 import org.meshtastic.core.repository.ServiceBroadcasts
 import org.meshtastic.core.repository.ServiceRepository
 import org.meshtastic.core.repository.StoreForwardPacketHandler
@@ -96,6 +97,7 @@ class MeshDataHandlerImpl(
     private val storeForwardHandler: StoreForwardPacketHandler,
     private val telemetryHandler: TelemetryPacketHandler,
     private val adminPacketHandler: AdminPacketHandler,
+    private val remoteShellHandler: RemoteShellHandler,
     @Named("ServiceScope") private val scope: CoroutineScope,
 ) : MeshDataHandler {
 
@@ -181,6 +183,12 @@ class MeshDataHandlerImpl(
                 adminPacketHandler.handleAdminMessage(packet, myNodeNum)
             }
 
+            PortNum.REMOTE_SHELL_APP -> {
+                remoteShellHandler.handleRemoteShell(packet)
+                // Do not broadcast — RemoteShell frames are point-to-point PTY I/O
+                shouldBroadcast = false
+            }
+
             PortNum.NEIGHBORINFO_APP -> {
                 neighborInfoHandler.handleNeighborInfo(packet)
                 shouldBroadcast = true

core/data/src/commonMain/kotlin/org/meshtastic/core/data/manager/RemoteShellPacketHandlerImpl.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2025-2026 Meshtastic LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.meshtastic.core.data.manager
+
+import co.touchlab.kermit.Logger
+import kotlinx.coroutines.flow.MutableSharedFlow
+import kotlinx.coroutines.flow.SharedFlow
+import kotlinx.coroutines.flow.asSharedFlow
+import org.koin.core.annotation.Single
+import org.meshtastic.core.model.util.decodeOrNull
+import org.meshtastic.core.repository.ReceivedShellFrame
+import org.meshtastic.core.repository.RemoteShellHandler
+import org.meshtastic.proto.MeshPacket
+import org.meshtastic.proto.RemoteShell
+
+/**
+ * Handles incoming [RemoteShell] packets (REMOTE_SHELL_APP portnum = 13).
+ *
+ * This is a scaffold implementation. The RemoteShell firmware feature is currently unreleased (gated to
+ * [org.meshtastic.core.model.Capabilities.supportsRemoteShell]). When the firmware ships, this handler should be
+ * expanded to manage PTY session state and relay I/O to the UI.
+ */
+@Single
+class RemoteShellPacketHandlerImpl : RemoteShellHandler {
+
+    /**
+     * Emits every received [ReceivedShellFrame] (decoded frame + sender node number).
+     *
+     * Uses [MutableSharedFlow] with a buffer so that rapid or structurally-identical frames are never silently dropped
+     * (unlike `StateFlow` which conflates by equality).
+     */
+    private val _lastFrame = MutableSharedFlow<ReceivedShellFrame>(extraBufferCapacity = 16)
+    override val lastFrame: SharedFlow<ReceivedShellFrame> = _lastFrame.asSharedFlow()
+
+    override fun handleRemoteShell(packet: MeshPacket) {
+        val payload = packet.decoded?.payload ?: return
+        val frame = RemoteShell.ADAPTER.decodeOrNull(payload, Logger) ?: return
+        Logger.d { "RemoteShell frame from ${packet.from}: op=${frame.op} sessionId=${frame.session_id}" }
+        _lastFrame.tryEmit(ReceivedShellFrame(from = packet.from, frame = frame))
+    }
+}