The file explorer on the phone

The other half of EXPLORER.md: a folder button on the session header opens
the machine's filesystem, starting where the session works.

It draws **over** the session in the same `Box`, so the session under it
stays composed -- its event stream keeps flowing, its draft and scroll
position stay where they were, and coming back from a file costs nothing.
Back steps one level inside it (editor, viewer, directory, parent) and only
closes from where it opened; the platform gesture, the button and the swipe
all go through the one function, so they cannot mean different things.

The viewer is a `LazyColumn` of lines rather than one `Text`, because text
layout is linear in the text and a twenty-thousand-line file in a single
`Text` measures all of it to draw a screenful. Lines do not wrap and share
one horizontal scroll, so a logical line is a visual line and the gutter
cannot come to number the wrong text; the gutter's width is measured from
the digit count of the line count in the style it is drawn in. The editor
is a `BasicTextField` with a `VisualTransformation` carrying the scanner's
spans, which is the one Compose API that colours a field's own text rather
than replacing the field.

`fileLanguage` reads the same table `fenceLanguage` does, so a language
added for fences is a language added for files.

A file that changed on the machine while it was open here refuses to be
overwritten and asks, with what each of the three answers costs. That is
the ordinary case, not the exotic one: an agent editing the file somebody
is reading is what this whole feature is for.

The speedometer moves off the header into the session settings dialog,
where the session's other about-the-session controls are, and the folder
takes a place between the usage chart and the cog -- widest scope to
narrowest, cog at the end, as Iris asked. Both benchmark scripts move onto
`ui-trace`'s new tap-by-label action in the same change, so the render
report is never unavailable and never pressed at a coordinate that has
stopped meaning anything; `app/bench-lib.sh` is what they share, and
`grep -n "tap [0-9]" app/*.sh` is the check.

Exercised on the emulator against the sandbox's new fixture tree, with a
screenshot or a ui-trace for each: the listing (dotfiles, directories
first, a symlink to a directory sorted with them, a name with a tab in it),
a highlighted file, binary, too big, a permission error, editing and
saving, the 409 and its Overwrite, back with unsaved edits, creating a name
that exists, creating one that does not and landing in the editor, an empty
directory, and `..` above the directory the session opened in.

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

app/androidApp/src/test/kotlin/com/example/aiapp/FileLinesTest.kt

@@ -0,0 +1,73 @@
+package com.example.aiapp
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+/**
+ * The line arithmetic behind the file viewer.
+ *
+ * Worth a test rather than an eye: a line number that is one out is invisible in a short file and
+ * obvious in a long one, and a colour that stops at a line break is invisible until the file has a
+ * block comment in it.
+ */
+class FileLinesTest {
+    @Test
+    fun `a file that ends with a newline has the number of lines its author would count`() {
+        assertEquals(listOf("one", "two"), FileLines.of("one\ntwo\n", null).lines)
+        assertEquals(listOf("one", "two"), FileLines.of("one\ntwo", null).lines)
+        // Only one is dropped: a blank line at the end of a file is a line somebody typed.
+        assertEquals(listOf("one", "two", ""), FileLines.of("one\ntwo\n\n", null).lines)
+    }
+
+    @Test
+    fun `an empty file is one empty line`() {
+        val lines = FileLines.of("", null)
+        assertEquals(1, lines.size)
+        assertEquals("", lines.line(0).text)
+    }
+
+    @Test
+    fun `a comment that spans lines is coloured on every line it covers`() {
+        val text = "fn a() {}\n/* still\n   a comment */\nfn b() {}\n"
+        val lines = FileLines.of(text, Language.RUST)
+        assertEquals(4, lines.size)
+        val comment = catppuccinSyntax().of(Kind.COMMENT)
+        // The whole of the middle line, and the part of the third up to the closer.
+        assertTrue(lines.line(1).spanStyles.any { it.item.color == comment && it.start == 0 })
+        val third = lines.line(2)
+        assertTrue(third.spanStyles.any { it.item.color == comment && it.end == third.length })
+        // And the code around it is not commented.
+        assertTrue(lines.line(0).spanStyles.none { it.item.color == comment })
+        assertTrue(lines.line(3).spanStyles.none { it.item.color == comment })
+    }
+
+    @Test
+    fun `a span never runs past the line it was cut into`() {
+        val lines = FileLines.of("val x = \"a\nb\"\nval y = 1\n", Language.KOTLIN)
+        for (index in 0 until lines.size) {
+            val line = lines.line(index)
+            assertTrue(
+                line.spanStyles.all { it.start >= 0 && it.end <= line.length },
+                "line $index",
+            )
+        }
+    }
+
+    @Test
+    fun `a file with no language is plain`() {
+        val lines = FileLines.of("fn main() {}\n", null)
+        assertTrue(lines.line(0).spanStyles.isEmpty())
+    }
+
+    @Test
+    fun `a language comes from the extension, and only from a real one`() {
+        assertEquals(Language.KOTLIN, fileLanguage("Main.kt"))
+        assertEquals(Language.KOTLIN, fileLanguage("build.gradle.kts"))
+        assertEquals(Language.RUST, fileLanguage("files.rs"))
+        assertEquals(Language.TOML, fileLanguage("Cargo.toml"))
+        assertEquals(null, fileLanguage("Makefile"))
+        assertEquals(null, fileLanguage(".bashrc"))
+        assertEquals(null, fileLanguage("notes.txt"))
+    }
+}