Draw an inline code chip behind the text instead of under it

The chip was the renderer's span background, and a span's background is
part of the text's own drawing: the text node paints the selection first
and the glyphs over it, so an opaque chip covered the selection and
selecting a sentence highlighted every word of it except the ones in
backticks. The previous fix let the selection show through by taking the
chip to 60% alpha, which is a compromise on both sides -- the chip is a
weaker step down from the page, and selected it reached #3C344F where the
words around it reached #776394.

There is a place that is under both, and a fenced block was already in it:
a modifier on the text rather than a style inside it. So `appendCodeChip`
takes the code span from the renderer's inline builder, keeps its style and
its space of padding either side but drops the background, and marks the
range; `LinkedText` draws those ranges in a `drawBehind`. The chip is back
to the full `rawSurface` fill (measured #11111B against a #1E1E2E page) and
a selection over it now lands at #776394, the same as the rest of the
sentence -- the fenced block's numbers exactly.

The geometry is one box per line, from the bounding boxes of the run's
first and last characters, taken as far as the line's `visibleEnd`. Not
`getPathForRange`: that is the shape of a *selection*, which runs to the
right edge of every line but the last, and a code span that wrapped left a
full-width empty chip behind on the line above -- twice in one fixture.
`visibleEnd` is the same rule the selection rectangle obeys, so the chip
stops where the selection stops instead of sticking its padding space out
past the end of a selected line.

Checked on the emulator against a fixture with chips in a heading, three
kinds of list item, a quote, a table cell and a link label, unselected and
under Select All, and a link with a chip in its label still opens. Cost,
against the same build without the change, streaming sixty paragraphs of
three chips each: measure 755ms against 776ms, record 327ms against 321ms,
transcript draw 0.22ms in both.

AGENTS.md

@@ -307,6 +307,10 @@ first if a remote spawn ever mangles an argument.
   reply into the tool output under it -- and a container per row leaves
   whatever was drawn without one silently unselectable, which nothing on
   screen reports. Rows keep their tap handlers; selection is a long press.
+  **An inline code chip is drawn behind the text** rather than as the
+  renderer's span background, because a span background is part of the
+  text's own drawing and hid the selection under it -- see
+  `appendCodeChip` in `MarkdownLinks.kt` and TRANSCRIPT_RENDERING.md.
 - **A session can be moved to another directory** from the settings dialog
   (`POST /sessions/{id}/cwd`). It stops the process, because a working
   directory is settled at spawn; the next message starts it in the new one.