test(terminal): add cell-grid verification via libghostty-vt

[nlopes]

Parse converter output through the real Ghostty VT emulator and assert
on the rendered cell grid (char, bold/italic, OSC 8 hyperlink) instead
of diffing raw ANSI byte streams. This layer survives harmless SGR
reorderings and actually exercises the rendering pipeline a terminal
user experiences, catching bugs where the bytes look right but would
render wrong.

The existing fixture-based integration tests are kept as-is; this is
additive. Tests live in converters/terminal/tests/grid_test.rs and
cover section headers, paragraphs, bold/italic inline formatting,
ordered/unordered lists, NOTE and WARNING admonitions, tables,
source blocks, and OSC 8 hyperlink attribution (both with the
capability forced on and off).

• Adds libghostty-vt 0.1.1 as a dev-dependency of the terminal crate.
• Adds Processor::with_capabilities() so tests can force OSC 8 on
  regardless of the host TERM environment (mirrors the existing
  with_terminal_width() builder pattern).
• Installs Zig 0.15.1 via mlugg/setup-zig in the test-converters,
  clippy, and doctest CI jobs, which are the jobs that compile the
  dev-dependency through --all-targets / --doc.
• Updates acdc-converters-terminal CHANGELOG.