chrome: reframe user prompts as a labelled chip + rail

The header was lost in 77f1d64: dropping the side rails also dropped
frameTop() from the return, leaving a bare closing rule under the message.
A rule *underneath* reads as an opener for the reply as much as a
terminator for the prompt, and it shared its glyph vocabulary with every
other box, so prompts were distinguished only by hue.

Replace it with a reverse-video chip and a left gutter:

  █ ❯ PROMPT ▌──────────────────────────────────
  ▌ the prompt text
  ▌ wrapped continuation

The chip is the only filled element in the chrome and the rail the only
vertical (rails were removed everywhere else), so prompts are now
identifiable at a glance without adding glyphs to the rest of the UI.
The left cap is a FULL block, not ▐: the rail fills the left half of
column 0, so a right-half cap would sit in the opposite half of the same
cell and read as detached.

No closing rule; the reply terminates the block.

The native component wraps text in Box(pad, 1, bg), i.e. a blank padded
line above and below with OSC 133 markers on the first and last line.
paddingY is hardcoded and not configurable, so strip both pad lines and
re-home the markers: ZONE_START onto the header, ZONE_END+ZONE_FINAL
APPENDED to the last text line (prefixing would put the final line of the
prompt outside the semantic zone). Terminal prompt-jumping keeps working.

Note the OSC payload is not numeric-only -- pi emits 133;A / 133;B / 133;C
-- so the matcher runs to the terminator rather than a [0-9;] class. An
earlier numeric-only class silently matched nothing and left the pad lines
in place.

No timestamp: UserMessageComponent receives only `text`, and stamping at
first render would relabel every restored message with the restore time
under `pi -c`.

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