suppress POSIX-shaped lint hits in libc FFI bindings

`lib/src/pty/ffi/libc.dart` carried 34 analyze infos:

- 26 × `non_constant_identifier_names` on struct field names
  (`msg_name`, `iov_base`, `msg_controllen`, etc.) that map 1:1
  to POSIX (`man 2 socketpair`, `recvmsg`, `iovec`, `msghdr`).
- 8 × `library_private_types_in_public_api` on the C / Dart
  function-signature typedefs (`_SocketpairC`, `_SocketpairDart`,
  etc.) consumed only by the `lookupFunction<...>()` calls in
  this same file.

Renaming the field names to lowerCamelCase would diverge from the
spec the file documents itself against; promoting the typedefs to
public would just add noise to the import surface. This is the
textbook FFI-binding case where the lints work against the file's
purpose.

Adds a file-wide `// ignore_for_file:` directive — explicitly
approved per the no-lint-suppression rule, with the reason
written inline above the directive so a future reader can
re-evaluate it.

Project analyze drops 65 → 31 issues.

Co-Authored-By: Claude <noreply@anthropic.com>

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-06T21:46:24Z",
+  "exported_at": "2026-05-06T21:46:36Z",
   "decisions": [
     {
       "id": "D-1",