Swapping the emulator behind the `Emulator` trait silently changes what
`expect`, `snapshot`, and the SVG renderer see, and the grid had no tests at
all. Add a macro-generated suite that every backend opts into with one line,
so a second backend is verified against the same contract as the first.

Covers grid shape, blank-vs-space cells, SGR attributes, palette/RGB/default
colors, wide-char spacers, cursor position and clamping, resize, scrollback
retention and ordering, alternate screen, erase, PTY write-back for DSR, and
escape sequences split across reads.

Each case is its own test, so a failure names the part of the contract that
broke. 17 cases pass against the alacritty backend.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

Review found several cases asserted far less than their names claimed, so a
materially broken backend could pass the suite that exists to gate backends:

- cursor clamping accepted any in-bounds value, including (0, 0); now pins the
  exact bottom-right cell
- DSR accepted any CSI prefix; now pins the exact cursor position report
- erase and blank cells checked rendered text only, so a backend that dropped
  colors or attributes passed; now compares whole cells against the default
- resize checked only the reported size, never that content survived
- full_rows compared lengths, not contents, and never pinned that history is
  followed by exactly the viewport, which `grid(full = true)` relies on

Also covers areas that were untested: split UTF-8 across reads (the reader
uses a fixed 8 KiB buffer, so real output splits codepoints), autowrap, tabs,
bare CR overwrite, backspace, erase-to-end-of-line, and the scrollback limit,
which no case exercised despite the daemon requesting 5000 rows.

Verified by mutation: a backend that reports the cursor at the origin, renders
blank cells as spaces, or drops scrollback now fails 8 cases; before it passed
all of them.

Tabs pin only cursor advance and landing column. Alacritty stores a literal
tab in the skipped cells and other emulators store blanks, so asserting the
rendered text there would fail a correct backend.

The macro no longer emits `use` statements into the caller's module; a second
backend whose test module imports Color or Emulator itself would have hit
E0252.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

The `cells` response emitted four of the seven boolean attributes the
neutral model carries. `dim`, `invisible` and `strike` were parsed, stored
and then dropped at the wire, and `blink` had nowhere to go at all.

A client should be written against the vocabulary, not against whichever
backend happens to be compiled in, so all seven are reported now. `blink`
is always false from alacritty, which parses SGR 5/6/25 and discards them,
but ghostty (`Style.Flags.blink`) and xterm.js (`FgFlags.BLINK`) both track
it and will fill it in without any client change.

Underline style and color reach the wire for the same reason, as
`underline_style` and `underline_color` alongside the existing boolean.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

The rationale on `NamedColor` claimed xterm.js carries only palette
indices. It does not: `CM_P16` and `CM_P256` are distinct color modes, so
xterm.js preserves `SGR 31` vs `SGR 38;5;1` exactly as alacritty does.

The reason to drop that distinction is ghostty, whose `Style.Color` has a
single `.palette` variant and cannot reproduce it, plus the fact that
nothing here consumes it: bold-to-bright keys off the index, not off how
the color was spelled.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

The monitor wrote ";58;2::{r}:{g}:{b}", which is neither valid form: the
semicolon ends the parameter, so a terminal reads 58 as a bare "set
underline color" and takes whatever follows as its arguments. The
foreground color emitted after it was swallowed whole, leaving the text
uncolored and the underline painted the color the text should have been.

SGR 58 takes colon-joined subparameters. Emit "58:2::{r}:{g}:{b}" and
"58:5:{n}", matching the ";4:{sub}" already emitted for the style.

Nothing caught this because sgr output only ever went to a real terminal,
where a malformed escape reassigns parameters silently instead of
failing. Feed it back through an emulator and compare the style that
comes out.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

underline_style was null when a cell had no underline and underline_color
was null when the underline followed the text color, so a client had to
null-check two fields whose neighbours, fg and bg, already spell the same
idea as the string "default".

Give both a value instead. An absent underline is the "none" style, and
an underline with no color of its own is "default", matching fg and bg.
A client can now switch on the style string and compare the color the
same way it compares any other, and the underline bool stays as a
shorthand for style != "none".

Move the style's wire name onto UnderlineStyle so the mapping lives with
the type the next backend will reuse rather than in the daemon.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

A cell that sets no color of its own matched no --fg or --bg value, on
purpose: which color it paints is the viewer's theme's choice, so --fg 0
claiming it is black would be a lie. That left no way to assert on it at
all, even though snapshots and failure messages had been calling it
"default" all along.

Accept that same word as an expected value. --fg default passes only for
a cell that set no foreground, and a cell that did set one now fails
against it, so the assertion reads both ways.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

The cell held Option<Underline { style, color }>, so "not underlined"
had two spellings and every reader went through a map or and_then to
reach either half. The daemon, the monitor, the SVG renderer and the
snapshot serializer all paid that toll, and the wire format already
disagreed with the model: it reports a "none" style and a "default"
color rather than nulls.

Store the shape and the color side by side instead. UnderlineStyle grows
a None variant and becomes its own Default, so the cell reads
`underline` and `underline_color` the same way it reads `fg` and `bg`,
and the wire form is now a direct projection of the model rather than a
translation of it.

This also fixes a quiet loss. The underline color was reachable only
through the underline, so a cell that set SGR 58 without SGR 4, or that
cleared the shape with SGR 24, reported no color at all. A terminal
tracks the two separately and only a full reset clears both; the flat
model can say so, and a conformance test now pins it.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

The wrap fix and the snapshot fix were each tested on their own: the grid
in the conformance suite, `serialize` on hand-built rows. Neither ran the
serializer over a grid a backend actually produced, which is precisely
the pairing that broke. Both halves were individually right and the frame
still came out a column wide, because they disagreed about what a
continuation renders as.

Run the real serializer against a real wrapped grid so a backend has to
get the composition right, not just its own half. Reintroducing either
bug fails this test.

Rename conformance_wrap_snapshot to conformance_wrap_grid_over_several_rows;
it dumps the grid and never touched the snapshot code its name claimed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

Snapshots stored a bare "is underlined", so a curly underline turning
single kept passing. The style name is recorded instead, matching the
vocabulary the daemon already reports in `underline_style`.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>