iris: a scroll area no longer opens at the end of content it has not measured

Scroll::content_len was 0.0 both for 'nothing here' and for 'not drawn
yet', so the first frame's clamp found a scroll range of zero, read
amt == len as 'sitting at the end' and set snap_end -- and the next
frame, now knowing the real length, jumped to it. On a phone that put a
code fence at the end of its longest line, mid-word, before anybody
touched it. It is an Option now, and the clamp does not answer a question
it cannot yet answer.

Which edge an area opens at is also a caller's decision rather than a
default: scrollable_on starts at the beginning (what is read),
scrollable_to_end pins to the end while content grows (what is typed --
the composer), both through one Scroll::new(inner, axis, at_end).

And tool.rs's raw_block pans sideways again: the 2026-09-06 'a
scrollable_on(Axis::X) around a non-editable Text draws nothing' defect
does not reproduce, most likely fixed by the shaped-mask work, so a long
command is readable rather than clipped.

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

docs/IRIS_TODO.md

@@ -801,18 +801,32 @@ has the fuller account.
   `reposition` fault f5b8893 fixed -- it survives that commit. The
   workaround costs the group the 4dp inset its Compose counterpart holds
   its cards off the edge by, so this is worth fixing.
-- [ ] **`scrollable_on(Axis::X)` on a non-editable `Text` draws nothing.**
+- [x] **`scrollable_on(Axis::X)` on a non-editable `Text` draws nothing.**
   The panel is drawn and the text inside it is not. A markdown fence does
   the same to a `TextEdit` and is fine, so it is the widget kind rather
   than the chain. `tool.rs`'s `raw_block` is `masked()` only until this is
   fixed, which means a long command is clipped rather than pannable.
+  **Not reproducible on 2026-09-08**: `raw_block` was changed to
+  `.scrollable_on(Axis::X).pad(..).masked_by(..)` and the command draws
+  normally (`IRIS_TOOLS_EXPANDED=1 iris/run-headless.sh transcript --shot`,
+  the `rm -rf target` card). Something between 09-06 and 09-08 fixed it --
+  the shaped-mask work (`.masked_by`, 38bf630) is the likeliest, since the
+  old chain was `.masked()` *inside* the padding. Left ticked with the
+  original symptom recorded rather than deleted, in case it comes back.
 - [ ] **No overflow ellipsis.** `TextAttrs` can wrap or not wrap; there is
   no "one line, ellipsised" the way `maxLines = 1` + `TextOverflow.
   Ellipsis` gives Compose. A tool card's summary is clipped instead, so
   nothing on screen says it was cut. Whichever end is cut has to be a
   choice when this lands: a path is identified by its tail, a command by
   its head.
-- [ ] **A drawn chevron.** `Chevron.kt` draws its own strokes precisely
+- [ ] **A drawn chevron.** *Still open, and now worse: the bundled fonts
+  were removed on 2026-09-07 in favour of the platform collection, so the
+  mark is a codepoint the phone's own faces may not have -- Iris's
+  2026-09-08 screenshot shows an empty box where it should be, and the
+  desktop render draws it as a small dot. UI_RULES: "don't rely on
+  characters the platform might not have." iris has rects, text and
+  textures, so the smallest honest fix is a generated texture rather than
+  a new primitive.* `Chevron.kt` draws its own strokes precisely
   because a chevron from a font is a glyph a system font may not have --
   and the bundled `NotoSans-Regular.ttf` indeed has no U+25B8/25BE/25B4,
   while `NotoSansMono-Regular.ttf` does. `tool.rs` sets the mark in the