fix: URL opener and +show-face output match Ghostty, register fixes

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

Cargo.toml

@@ -12,6 +12,8 @@ path = "src/main.rs"
 # Mirrors upstream's `-Dsentry` build option (off by default on Linux):
 # enables the local crash-report handler (`ghostty-crash`).
 sentry = ["ghostty-crash/sentry"]
+# Mirrors upstream's `-Dsnap` build option (off by default).
+snap = ["ghostty-os/snap"]
 
 [dependencies]
 ghostty-core.workspace = true