Resolves the parity items of docs/rewrite/notes/review-rv15-codex.md
(final-Z18.md):
- os::open inherits stdin like Zig's Child.init, strips LD_LIBRARY_PATH
only in Snap builds (new `snap` cargo feature, off by default, like
Zig's snap build option) and bounds output collection to 50 KiB per
stream (StdoutStreamTooLong/StderrStreamTooLong).
- +show-face writes diagnostics and lookup lines in the exact fragments
Zig's formatter produces, so buffer drains happen at the same byte
positions (0 diffs vs /usr/bin/ghostty over 0-44 bad options for
three codepoints); its tests no longer parse the test runner's argv.
- DEVIANT_FIXES.md: five entries reworded and four removed that were no
longer deviations (verified against code and oracle).
Test Plan:
- cargo test -p ghostty-os open; cargo test -p ghostty-cli show_face
