Support configured ASCII input delimiter

[MrAlaskan]

Summary

This PR fixes FramerAscii so that Change ASCII Input Delimiter actually affects future ASCII messages.

Previously, the delimiter value was updated in the control block, but the ASCII framer still used a hardcoded CRLF terminator for both encoding and decoding.

According to the Modbus ASCII specification, the Change ASCII Input Delimiter diagnostic command replaces the default LF character used at the end of future messages.

Change

• FramerAscii now builds its end-of-frame marker from the control block delimiter instead of always using a hardcoded CRLF
• decode() now looks for the configured delimiter
• encode() now emits the configured delimiter
• The control block default delimiter is set to LF, so the default end-of-frame remains CRLF

Rationale

This aligns the framer behavior with both the protocol and the existing diagnostic command implementation.

Without this change, Change ASCII Input Delimiter updates internal state but does not affect actual frame parsing or frame generation, so the command appears to succeed while the wire behavior remains unchanged.