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/test/java/com/m57/hermescontrol/ui/chat/tool/render/FileEditSupportTest.kt

@@ -0,0 +1,54 @@
+package com.m57.hermescontrol.ui.chat.tool.render
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class FileEditSupportTest {
+    @Test
+    fun testCountDiffLineStats_emptyDiff() {
+        val stats = FileEditSupport.countDiffLineStats("")
+        assertEquals(0, stats.added)
+        assertEquals(0, stats.removed)
+    }
+
+    @Test
+    fun testCountDiffLineStats_headerFiltering() {
+        val diff =
+            """
+            --- a/file.txt
+            +++ b/file.txt
+            @@ -1,5 +1,5 @@
+             unchanged
+            -removed line
+            +added line
+            """.trimIndent()
+        val stats = FileEditSupport.countDiffLineStats(diff)
+        assertEquals(1, stats.added)
+        assertEquals(1, stats.removed)
+    }
+
+    @Test
+    fun testCountDiffLineStats_crlfHandling() {
+        val diff = "---\r\n+++\r\n+added\r\n-removed\r\n+another\r\n"
+        val stats = FileEditSupport.countDiffLineStats(diff)
+        assertEquals(2, stats.added)
+        assertEquals(1, stats.removed)
+    }
+
+    @Test
+    fun testCountDiffLineStats_largeDiff() {
+        val sb = java.lang.StringBuilder()
+        sb.append("--- a/file.txt\n")
+        sb.append("+++ b/file.txt\n")
+
+        val count = 5000
+        for (i in 0 until count) {
+            sb.append("-old line $i\n")
+            sb.append("+new line $i\n")
+        }
+
+        val stats = FileEditSupport.countDiffLineStats(sb.toString())
+        assertEquals(count, stats.added)
+        assertEquals(count, stats.removed)
+    }
+}