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/stream-bench.sh

@@ -4,8 +4,9 @@
 # reply that is still arriving cheap", the way transcript-bench.sh is for
 # scrolling one that has settled.
 #
-#   ./stream-bench.sh FILE           restart the app, open the first
-#                                    session, stream FILE into it, report
+#   ./stream-bench.sh FILE           restart the app, open the session the
+#                                    sandbox lists first, stream FILE into
+#                                    it, report
 #   ./stream-bench.sh -k FILE        keep whatever session is open now
 #
 # The session is the first one the sandbox lists (ui-sandbox.sh spawn makes
@@ -17,6 +18,7 @@
 set -eu
 cd "$(dirname "$0")"
 . ./android-env.sh >/dev/null 2>&1
+. ./bench-lib.sh
 
 keep=""
 while getopts k flag; do
@@ -31,11 +33,9 @@ file=${1:?usage: stream-bench.sh [-k] FILE}
 sid=$(./ui-sandbox.sh api /sessions | python3 -c 'import json,sys; print(json.load(sys.stdin)[0]["id"])')
 
 if [ -z "$keep" ]; then
-    adb shell am force-stop com.example.aiapp
-    adb shell am start -n com.example.aiapp/.MainActivity >/dev/null
-    sleep 5
-    ui-trace record -d 3000 --do 'tap 500 545' -o /tmp/bench-open.txt >/dev/null 2>&1
-    sleep 3
+    title=$(first_session_title)
+    [ -n "$title" ] || { echo "stream-bench: no sessions to open" >&2; exit 1; }
+    open_session "$title"
 fi
 
 # Pinned to the newest end before anything is sent. The transcript never
@@ -46,18 +46,17 @@ fi
 # reads exactly like a build where the work vanished. The control is the
 # app's own "Jump to latest", which is only there while the newest message
 # is off screen -- so when it is absent the list is already where it needs
-# to be.
-jump=$(ui-trace record -d 1200 -o /tmp/bench-jump.txt >/dev/null 2>&1
-    ui-trace show /tmp/bench-jump.txt -m 'Jump to latest' --field box |
-        grep -o '[0-9]*,[0-9]*\.\.[0-9]*,[0-9]*' | tail -1)
-if [ -n "$jump" ]; then
-    x=$(echo "$jump" | awk -F'[,.]' '{print int(($1 + $4) / 2)}')
-    y=$(echo "$jump" | awk -F'[,.]' '{print int(($2 + $5) / 2)}')
-    ui-trace record -d 1500 --do "tap $x $y" -o /tmp/bench-tolatest.txt >/dev/null 2>&1
+# to be. Looked for before it is pressed rather than pressed blindly: a tap
+# by name fails the whole run when its label is missing, and missing is the
+# ordinary case here.
+ui-trace record -d 1200 -o /tmp/bench-jump.txt >/dev/null 2>&1
+if ui-trace show /tmp/bench-jump.txt -m 'Jump to latest' --field box | grep -q '[0-9],[0-9]'; then
+    ui-trace record -d 2500 --do "tap 'Jump to latest'" --do 'wait 1000' \
+        -o /tmp/bench-tolatest.txt >/dev/null
 fi
 
-# The first tap resets the report's window; see transcript-bench.sh.
-ui-trace record -d 2000 --do 'tap 723 205' -o /tmp/bench-reset.txt >/dev/null 2>&1
+# The first copy resets the report's window; see transcript-bench.sh.
+copy_render_report /tmp/bench-reset.txt
 adb logcat -c
 
 ./ui-sandbox.sh send "$sid" "@$file" >/dev/null
@@ -90,6 +89,6 @@ while [ "$i" -lt 180 ]; do
 done
 sleep 1
 
-ui-trace record -d 2000 --do 'tap 723 205' -o /tmp/bench-report.txt >/dev/null 2>&1
+copy_render_report /tmp/bench-report.txt
 sleep 1
 adb logcat -d -s ai-app:I | sed -n '/ai-app render report/,$p' | sed 's/^.*ai-app  : //'