Draw verbatim text on its own dark surface

A command, a tool's output and a code block in a reply are the one thing on
this screen that is not somebody's prose, and they now say so: Mocha's
Crust, which sits below Base, so the same colour is one clear step down both
on the page where a reply is drawn and on the card where a tool call is.

The renderer's code background was `surfaceVariant`, which is exactly a
card's own fill -- a fenced block inside a tool call had no background at
all, and one in a reply read as a step *up* out of the page.

Tool output takes the monospace face with it. It is column-aligned far more
often than it is prose -- a listing, a diff, a table of numbers -- and a
proportional font silently destroys the alignment that carried the meaning.

`RawBlock` is a composable rather than a modifier because the inset is part
of it: monospace text against the edge of a tinted block reads as clipping.
A call with neither a subject nor any other field draws nothing at all
rather than an empty tinted rectangle.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/androidApp/src/main/kotlin/com/example/aiapp/RawBlock.kt

@@ -0,0 +1,38 @@
+package com.example.aiapp
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.ColumnScope
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.unit.dp
+
+/**
+ * Verbatim text, on the surface that says so: a command about to be run, what a tool printed.
+ *
+ * A composable rather than a modifier repeated at each site, because the inset is part of it --
+ * monospace text drawn hard against the edge of a tinted block reads as a clipping fault, and three
+ * copies of "clip, fill, pad" drift apart the first time one of them is adjusted.
+ *
+ * The colour is [rawSurface], which is also what a code block inside a reply is given; that is the
+ * point of having one name for it. Markdown's blocks are painted by the renderer rather than by
+ * this, since it draws its own, but they are the same colour on purpose.
+ */
+@Composable
+fun RawBlock(modifier: Modifier = Modifier, content: @Composable ColumnScope.() -> Unit) {
+    Column(
+        modifier
+            .fillMaxWidth()
+            // Smaller than a card's radius, and deliberately: this sits *inside* one, and a
+            // rounded rectangle drawn at the same radius as the rounded rectangle behind it reads
+            // as a misprint rather than as nesting.
+            .clip(MaterialTheme.shapes.extraSmall)
+            .background(rawSurface)
+            .padding(horizontal = 8.dp, vertical = 6.dp),
+        content = content,
+    )
+}