Fix GUI paste of CRLF/CR text: split all line endings

The ImGui Cmd+V/Ctrl+V paste handler split clipboard text only on '\n',
leaving '\r' inside each segment. Pasting text with CRLF (Windows) or bare
CR (classic-Mac / some macOS apps) line endings enqueued an InsertText arg
containing '\r', which the InsertText command rejects with "InsertText arg
must not contain newlines".

Extract the paste-to-commands logic into a pure, SDL-free helper
(PasteSplit.h) that treats '\n', '\r\n', and bare '\r' each as one line
break, and add unit tests covering all line-ending forms plus blank-line
and trailing-newline preservation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

PasteSplit.h

@@ -0,0 +1,48 @@
+/*
+ * PasteSplit.h - split pasted/clipboard text into editor commands
+ *
+ * Pure logic (no SDL/GUI dependencies) so it can be unit tested and shared
+ * across frontends. Line breaks in the pasted text must become Newline
+ * commands; InsertText itself rejects embedded '\r'/'\n' (see Command.cc).
+ */
+#pragma once
+#include <string>
+#include <vector>
+
+#include "InputHandler.h"
+
+
+// Translate a block of pasted text into a sequence of editor commands.
+//
+// Any of "\n", "\r\n", or a bare "\r" is treated as a single line break and
+// emitted as a Newline command; the text between breaks becomes InsertText
+// commands. Empty text segments are skipped, but line breaks are always
+// emitted so blank lines round-trip correctly.
+inline std::vector<MappedInput> SplitPasteIntoCommands(const std::string &text)
+{
+	std::vector<MappedInput> out;
+	std::string segment;
+
+	auto flush_segment = [&]() {
+		if (!segment.empty()) {
+			out.push_back(MappedInput{true, CommandId::InsertText, segment, 0});
+			segment.clear();
+		}
+	};
+
+	for (std::size_t i = 0; i < text.size(); ++i) {
+		const char c = text[i];
+		if (c == '\n' || c == '\r') {
+			flush_segment();
+			out.push_back(MappedInput{true, CommandId::Newline, std::string(), 0});
+			// Collapse a "\r\n" pair into one line break.
+			if (c == '\r' && i + 1 < text.size() && text[i + 1] == '\n')
+				++i;
+		} else {
+			segment.push_back(c);
+		}
+	}
+	flush_segment();
+
+	return out;
+}