Fix: Avoid eager List allocation in countDiffLineStats for large diffs

Replaced `diff.split("\n")` with `diff.lineSequence()` in `FileRenderers.kt`'s `countDiffLineStats()` method. This prevents massive `ArrayList` allocations on the heap when processing multi-thousand-line diffs during tool card rendering, improving memory efficiency by evaluating the lines lazily.

Added comprehensive unit tests in `FileEditSupportTest.kt` covering empty diffs, header filtering, CRLF handling, and large multi-line diffs.

app/src/main/java/com/m57/hermescontrol/ui/chat/tool/render/FileRenderers.kt

@@ -79,7 +79,7 @@ internal object FileEditSupport {
         var added = 0
         var removed = 0
 
-        for (line in diff.split("\n")) {
+        for (line in diff.lineSequence()) {
             when {
                 line.startsWith("+") && !line.startsWith("+++") -> added += 1
                 line.startsWith("-") && !line.startsWith("---") -> removed += 1