refactor(companion): extract MessageHistory store from QuickMsgScreen

Move the two RAM history rings (channel + DM) out of the 1988-line
QuickMsgScreen into a self-contained MessageHistory.h component, mirroring
the WaypointsView extraction from TrailScreen. The store owns the ring
buffers, per-entry delivery state (channel relay echo, DM end-to-end ACK +
auto-resend) and the per-channel unread counters; the screen keeps all
view state (selection, scroll, fullscreen readers, the unread "viewing
session" bookkeeping) and reaches entries through accessors.

Behaviour-preserving:
- The shared types (AckState, ChHistEntry, DmHistEntry, MSG_TEXT_BUF) are
  file-scope in MessageHistory.h, so the phase machine still refers to them
  unqualified — only data + storage logic moved.
- addChannelMsg keeps a thin screen forwarder that computes the "viewing"
  flag (a phase fact the store can't see) and returns the ring pos;
  afterSend uses armChannelRelay(pos, seq) instead of poking the ring.
- The public API called via UITask/MyMesh/the bot (addChannelMsg, addDMMsg,
  markDmDelivered, markChannelRelayed, tickDmResends, clearAllChannelUnread,
  getTotalChannelUnread, getRecentDMContacts) stays on QuickMsgScreen as
  forwarders, so no caller changes.

QuickMsgScreen 1988 -> 1738 lines; MessageHistory.h 324. Both solo boards
build. Single-TU fragment: included by UITask.cpp before QuickMsgScreen.h.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.cpp

@@ -130,6 +130,7 @@ static const int QUICK_MSGS_MAX = 10;
 #include "FullscreenMsgView.h"
 #include "SensorPlaceholders.h"
 #include "SettingsScreen.h"
+#include "MessageHistory.h"   // RAM history rings (DM + channel) used by QuickMsgScreen
 #include "QuickMsgScreen.h"
 
 // ── Custom screens (separate files to ease upstream merges) ───────────────────