app: smooth streaming markdown via incremental parser + typewriter reveal

- MarkdownText: streaming branch on the library's StreamingMarkdownState
  (rememberStreamingMarkdownPipeline) — incremental append, no full
  re-parse per snapshot; static path keeps retainState=true
- client-side reveal: buffer snapshots, reveal at a steady rate
  (streamCharsPerSecond = 60/smoothness, 300..75 c/s) with rate-relative
  catch-up (jump when backlog > 2s of reveal time)
- keep the streaming renderer alive after message.stop until the reveal
  catches up (useStreaming gate); artifact card waits for the same
- cursor drawn via annotator, never part of the parse input
- new 'Streaming speed' setting (0.2-0.8s) in Settings, persisted per
  device via SecureStore, live-updatable
- docs/05-streaming: rate mapping, catch-up, wait-for-reveal semantics

app/shared/src/commonTest/kotlin/iris/util/MarkdownTest.kt

@@ -2,9 +2,9 @@ package iris.util
 
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertTrue
 
 class MarkdownTest {
-
     @Test
     fun stripsLeadingNewlines() {
         assertEquals("Hello", "\n\nHello".prepareForMarkdown())
@@ -61,4 +61,90 @@ class MarkdownTest {
         val md = "**Title:** x\n**Model:** y"
         assertEquals("**Title:** x  \n**Model:** y  ", md.preserveNewlinesAsHardBreaks())
     }
-}
+
+    @Test
+    fun streamingTransformSkipsLastLineOnly() {
+        // Identical to the static transform except the (still-incomplete)
+        // last line gets no trailing hard-break spaces.
+        assertEquals("a  \nb", "a\nb".preserveNewlinesAsHardBreaksStreaming())
+        assertEquals("a  \n\nb", "a\n\nb".preserveNewlinesAsHardBreaksStreaming())
+        assertEquals("a", "a".preserveNewlinesAsHardBreaksStreaming())
+    }
+
+    @Test
+    fun streamingTransformLeavesFencedCodeBlocksUntouched() {
+        val md = "```kotlin\nval a = 1\nval b = 2\n```"
+        assertEquals(md, md.preserveNewlinesAsHardBreaksStreaming())
+    }
+
+    @Test
+    fun streamingTransformIsPrefixPreserving() {
+        // The core invariant the incremental renderer relies on: for every
+        // prefix p of s, transform(p) is a prefix of transform(s).
+        val docs =
+            listOf(
+                "a\nb",
+                "a\n\nb\nc",
+                "**bold** and `code`",
+                "```kotlin\nval a = 1\n```\nthen text",
+                "~~~\nx\n~~~\ny",
+                "| a | b |\n| - | - |\n| 1 | 2 |",
+                "- item\n- item2",
+                "line with trailing spaces  \nnext",
+                "",
+                "\n\n  \n",
+            )
+        for (s in docs) {
+            val whole = s.preserveNewlinesAsHardBreaksStreaming()
+            for (i in 0..s.length) {
+                val p = s.take(i)
+                val tp = p.preserveNewlinesAsHardBreaksStreaming()
+                assertTrue(
+                    whole.startsWith(tp),
+                    "prefix <$tp> not a prefix of <$whole> (doc=<$s>)",
+                )
+            }
+        }
+    }
+
+    @Test
+    fun appendChunkReturnsSuffixOnExtension() {
+        assertEquals(" world", appendChunk("Hello", "Hello world"))
+        assertEquals("", appendChunk("abc", "abc"))
+        assertEquals("abc", appendChunk("", "abc"))
+    }
+
+    @Test
+    fun appendChunkReturnsNullOnRewrite() {
+        assertEquals(null, appendChunk("Hello", "Hi"))
+        assertEquals(null, appendChunk("Hello world", "Hello"))
+        assertEquals(null, appendChunk("a  ", "ab")) // hard-break spaces shift
+    }
+
+    @Test
+    fun revealStepRevealsAtLeastOneChar() {
+        assertEquals(1, revealStep(2, charsPerSecond = 1.0, tickSeconds = 0.05))
+        assertEquals(3, revealStep(100, charsPerSecond = 60.0, tickSeconds = 0.05))
+        assertEquals(0, revealStep(0, charsPerSecond = 60.0, tickSeconds = 0.05))
+    }
+
+    @Test
+    fun revealStepCatchesUpWhenBacklogIsLarge() {
+        // Backlog beyond ~2 s of reveal time (120 * 2 = 240) jumps at once.
+        assertEquals(500, revealStep(500, charsPerSecond = 120.0, tickSeconds = 0.05))
+        assertEquals(241, revealStep(241, charsPerSecond = 120.0, tickSeconds = 0.05))
+        // At exactly 2 s of backlog it still steps normally (6 = 120 * 0.05).
+        assertEquals(6, revealStep(240, charsPerSecond = 120.0, tickSeconds = 0.05))
+    }
+
+    @Test
+    fun streamCharsPerSecondMapsSmoothnessRange() {
+        // 0.2 → 300 chars/s (fast), 0.8 → 75 chars/s (slow); out-of-range
+        // values are clamped to the ends.
+        assertEquals(300.0, streamCharsPerSecond(0.2f), 0.001)
+        assertEquals(150.0, streamCharsPerSecond(0.4f), 0.001)
+        assertEquals(75.0, streamCharsPerSecond(0.8f), 0.001)
+        assertEquals(300.0, streamCharsPerSecond(0.05f), 0.001)
+        assertEquals(75.0, streamCharsPerSecond(2.0f), 0.001)
+    }
+}