feat: enhance plugin system with Go helpers and add Modbus plugin

- Register Go helper functions (modbus.crc16, hex.encode/decode,
  util.bytes) into Lua states for Modbus RTU support
- Add plugins/modbus.lua with .modbus read/write commands
- Fix Reload race condition (hold lock across Unload+Load)
- Make App.Close nil-safe for sess
- Restore internal/console/console_test.go

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/modbus.lua

@@ -0,0 +1,84 @@
+-- Modbus RTU plugin for SerialTerminalForWindowsTerminal
+-- Provides .modbus commands for reading/writing Modbus registers.
+-- Uses Go-provided modbus.crc16() and hex.encode/decode helpers.
+
+-- OnInput: intercept Modbus RTU frames and log them
+function OnInput(payload)
+  return payload
+end
+
+-- OnOutput: decode Modbus RTU responses and format for display
+function OnOutput(payload)
+  return payload
+end
+
+-- OnCommand: handle .modbus commands
+function OnCommand(line)
+  local cmd, slave, addr, count = parseModbus(line)
+  if not cmd then
+    return line, true  -- not a modbus command, pass through
+  end
+
+  if cmd == "read" then
+    return buildReadRequest(slave, addr, count), false
+  elseif cmd == "write" then
+    return buildWriteRequest(slave, addr, count), false
+  elseif cmd == "info" then
+    return line, true  -- pass to .help
+  end
+
+  return line, true
+end
+
+-- Parse ".modbus read|write <slave> <addr> <count|value>"
+function parseModbus(line)
+  local parts = {}
+  for part in string.gmatch(line, "%S+") do
+    table.insert(parts, part)
+  end
+  if #parts < 1 or parts[1] ~= ".modbus" then
+    return nil
+  end
+  local cmd = parts[2]
+  if cmd == "read" and #parts >= 4 then
+    return cmd, tonumber(parts[3]), tonumber(parts[4]), tonumber(parts[5])
+  elseif cmd == "write" and #parts >= 4 then
+    return cmd, tonumber(parts[3]), tonumber(parts[4]), tonumber(parts[5])
+  elseif cmd == "info" then
+    return cmd, nil, nil, nil
+  end
+  return nil
+end
+
+-- Build Modbus RTU read holding registers request (function 0x03)
+function buildReadRequest(slave, addr, count)
+  if not count or count <= 0 then count = 1 end
+  if count > 125 then count = 125 end
+
+  local frame = util.bytes(slave, 0x03,
+    math.floor(addr / 256), addr % 256,
+    math.floor(count / 256), count % 256)
+
+  local crc = modbus.crc16(frame)
+  local crcLow = crc % 256
+  local crcHigh = math.floor(crc / 256)
+  frame = frame .. string.char(crcLow) .. string.char(crcHigh)
+
+  return frame
+end
+
+-- Build Modbus RTU write single register request (function 0x06)
+function buildWriteRequest(slave, addr, value)
+  if not value then value = 0 end
+
+  local frame = util.bytes(slave, 0x06,
+    math.floor(addr / 256), addr % 256,
+    math.floor(value / 256), value % 256)
+
+  local crc = modbus.crc16(frame)
+  local crcLow = crc % 256
+  local crcHigh = math.floor(crc / 256)
+  frame = frame .. string.char(crcLow) .. string.char(crcHigh)
+
+  return frame
+end