feat: v0.4.0 performance & stability overhaul

Terminal:
- cache font variants + single reusable brush in render hot path;
  resolve cell colors once per cell; dispose GDI resources
- scrollback: List+RemoveRange -> O(1) ring buffer
- follow new output only when already at bottom (no more yank-to-bottom
  while reading history)
- alt-screen mouse wheel -> arrow keys (vim/htop scroll)
- answer DSR (ESC[5n/6n) and DA (ESC[c) queries so TUIs no longer hang
- remove dead code in OnKeyPress

Encoding correctness (garbled CJK across chunk boundaries):
- stateful UTF-8 Decoder in TTLInterpreter.OnData, SerialBridgeServer
  rx forwarding, and SessionLogger.Write

Sessions:
- ShellChannel: free proc-thread attribute list, close hProcess,
  notify '[ETTerms] shell process exited' in the tab
- SshChannel: surface ErrorOccurred / ShellStream.Closed in the tab;
  Write no longer throws into the UI thread on a dead connection

PDU:
- new shared ETTerms.PduCore project replaces the two drifted copies of
  PduController (GUI + PduMcp); logging via injected delegates
- batched SNMP GET (GetAllPortsStatus): 12-port poll is 3 UDP
  round-trips instead of 36 (StatusView polling + pdu_status tool)

Scripting:
- cap TTL receive buffer at 1MB; skip re-scan in wait when buffer
  length unchanged
- merge ScriptRunner.RunAsync/RunGroupAsync; new TtlScript helper
  dedups script picking + group-command checks (3 copies -> 1)

Misc:
- ConnectionStore: parse LastUsedUtc with InvariantCulture/RoundtripKind
- version 0.4.0; About changelog; CLAUDE.md notes (intentional group
  barrier behavior, SSH.NET reflection resize caveat)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ETTerms.PduCore/ETTerms.PduCore.csproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <RootNamespace>ETTerms.PduCore</RootNamespace>
+    <AssemblyName>ETTerms.PduCore</AssemblyName>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="SnmpSharpNet" Version="0.9.7">
+      <NoWarn>NU1701</NoWarn>
+    </PackageReference>
+  </ItemGroup>
+
+</Project>