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

crates/ghostty-os/Cargo.toml

@@ -51,6 +51,9 @@ crossbeam-channel = "0.5.17"
 [features]
 # Mirrors Zig's `wasm_shared` build option (SharedArrayBuffer-backed memory).
 wasm-shared = []
+# Mirrors Zig's `snap` build option (`-Dsnap`, off by default): the URL
+# opener drops `LD_LIBRARY_PATH` from the child environment.
+snap = []
 
 # Same as `[workspace.lints]` except `unsafe_code` is denied instead of
 # forbidden: the wasm JS import blocks (`src/wasm`) need `unsafe`.