iris: a Rect is not size-independent, and P1a's block appearance verified

The defect P1a's screenshots found, and the one that mattered:
`Rect::is_size_independent()` answered `true`. A `Rect` fills whatever
region it is handed, so its content *is* the region -- and
`draw_inner`'s fast path, which rewrites a widget's primitives with
`r.outside(&from).within(&region)` instead of redrawing it, cannot
reproduce that once a region carries both `rel` and `abs`. What it
looked like: a fenced code block's background kept the height of the
provisional full-region draw `Span` does in its first phase, so one
fence's panel covered every block below it and every row below that,
with the text underneath laid out correctly. Likely the same cause as
RUST.md's older "the composer bar's grey background is not drawn".

Also here: a quote's bar is a `Stack` background behind padded text
rather than a two-child `Span(Dir::RIGHT)` (one widget fewer and no
provisional pass), and `transcript-ui`'s `transcript` example gains a
row holding one of every block kind -- the fixture's own heading,
paragraph, fence and table source, plus a list and a quote, which the
fixture has neither of.

docs/bench/p1a-2026-09-06/ has the pairs and docs/RUST.md's P1a box
names what still differs. The iris half is from the desktop backend
because this emulator cannot draw iris's glyphs at all (solid boxes,
reproduced on the previous commit, with Compose drawing text correctly
on the same AVD); both routes to Vulkan on this AVD were tried and both
fail. Bench stream phase, assertions live, no abort: p50 53.0ms p90
108.6ms p99 132.0ms against 52.8/108.1/137.3 before -- unchanged.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/DECISIONS.md

@@ -5,6 +5,39 @@ they can be judged and reversed later. Detail lives in RUST.md (and IRIS.md
 for iris API changes); this file is only the summary. Newest first. Items
 marked **DEFERRED** are ones the agent chose not to decide alone.
 
+## 2026-09-06 (how a markdown block looks, P1a)
+
+- **A table is drawn as padded monospace columns, not as a grid.** Your
+  call to reverse. Compose draws a real grid: cells on a tint, each
+  column with a 136dp floor, scrolling sideways when there are too many.
+  iris has no grid widget, and building one would be a widget per
+  markdown feature -- which is the thing the block model exists to avoid.
+  In a monospace face a character count *is* a pixel width, so padding
+  each cell to its column's width is alignment, the widths are still
+  measured from the cells, and a table that is too wide pans sideways
+  through the same mechanism a code fence already uses. The header is
+  bold with a rule under it, and a long cell wraps inside its column
+  (capped at 28 characters, which is what fits three columns across a
+  phone). **What it trades:** no cell borders, and a table looks like
+  code rather than like a table. If you want the grid, it is a new widget
+  and it is a day's work.
+- **Three block frames, and only three.** A heading, paragraph and list
+  are plain text with spans; a fence and a table are a rounded panel that
+  does not wrap; a quote is a bar with the text padded past it.
+  Everything else markdown says is expressed in span styles, which cost
+  no widgets and no layout nodes. So a new markdown feature is a span,
+  not a widget.
+- **A list's marker is part of the text, so a wrapped item's second line
+  returns to the left margin.** Compose keeps it indented by giving the
+  marker its own column. Doing the same here needs per-line indent in
+  iris's text attributes; it is written down rather than done, because
+  the list items in a real reply are usually one line.
+- **A link opens on a tap and not on the end of a drag.** A press that
+  panned the transcript past a link, or that held long enough to start a
+  selection, does not follow it -- decided by the same gesture machine
+  that decides pan-versus-select, so there is one rule rather than two
+  that can disagree.
+
 ## 2026-09-06 (composer scroll and the streaming block model)
 
 - **A streamed message becomes a column of per-block widgets.** Decided by