test(core): add headless end-to-end tests and fix bugs they found

Adds crates/ghostty-core/tests/e2e_*.rs: 81 tests that drive the real
core Surface, termio, pty and child processes through a fake runtime
(recording apprt actions, in-memory clipboard) with a headless renderer
(ghostty-renderer/src/headless.rs). They cover keys (legacy, DECCKM,
kitty, modifyOtherKeys), keybinding actions and sequences, mouse
selection/reporting/scrolling/links, clipboard incl. OSC 52 and paste
safety, resize, focus, titles/pwd/bell/notifications/progress, child
exit, and IME preedit.

Fixes for issues found by the tests and by the Codex audit
(docs/rewrite/notes/review-core-codex.md, all 9 items):
- replacing a selection with copy-on-select panicked (stale tracked
  handles); global new_window no longer inherits a parent; the child
  environment uses the runtime's env wholesale so GTK's removals stick;
  Flatpak host resources path; locale recovery (ensureLocale) in
  apprt-gtk before GTK init and for CLI actions; failed screen-file
  opens propagate errors; OSC 52 base64 rules, CSI/ESC binding length
  limits and GHOSTTY_LOG parsing match Zig.
- Also includes V2's termio cleanups (tmux viewer wiring in the stream
  handler, Pty slave semantics docs, shared cgroup wait loop).

apprt-gtk gains a debug input replay hook (GHOSTTY_DEBUG_INPUT_SCRIPT)
exercising the real GTK key path. With it, the real app was verified on
Wayland: typing and output, new tab, splits, select-all/copy/paste and
font size all work (docs/rewrite/notes/verification-V1.md). Not yet
exercised at GTK level: mouse, real IM-context IME, clipboard dialogs.

Test Plan:
- cargo test --workspace (2983 passed)
- cargo test -p ghostty-core --tests (81 e2e tests)

crates/ghostty-core/src/surface/clipboard.rs

@@ -43,13 +43,19 @@ impl Surface {
             return;
         }
 
-        // Decode. Zig tolerates a missing-padding error ("It is possible to
-        // actually have valid data and get this error, so we allow it"), so
-        // decode leniently without requiring padding.
+        // Zig validates the size first (`calcSizeForSlice` on the padded
+        // standard alphabet rejects lengths that aren't a multiple of 4),
+        // then decodes ignoring only the `InvalidPadding` error (e.g. nonzero
+        // trailing bits as in "YR==", which still decodes to "a").
+        if !data.len().is_multiple_of(4) {
+            log::info!("application sent invalid base64 data for OSC 52");
+            return;
+        }
         let lenient = base64::engine::GeneralPurpose::new(
             &base64::alphabet::STANDARD,
             base64::engine::GeneralPurposeConfig::new()
-                .with_decode_padding_mode(base64::engine::DecodePaddingMode::Indifferent),
+                .with_decode_padding_mode(base64::engine::DecodePaddingMode::RequireCanonical)
+                .with_decode_allow_trailing_bits(true),
         );
         let Ok(decoded) = lenient.decode(data) else {
             log::info!("application sent invalid base64 data for OSC 52");
@@ -78,23 +84,8 @@ impl Surface {
 
         // The options we'll use for all formatting. We'll just override the
         // emit format.
-        let codepoint_map: Vec<formatter::CodepointMap> = cfg
-            .clipboard_codepoint_map
-            .map
-            .list
-            .iter()
-            .map(|e| formatter::CodepointMap {
-                range: e.range,
-                replacement: match &e.replacement {
-                    ghostty_config::clipboard_codepoint_map::Replacement::Codepoint(c) => {
-                        formatter::Replacement::Codepoint(*c)
-                    }
-                    ghostty_config::clipboard_codepoint_map::Replacement::String(s) => {
-                        formatter::Replacement::String(s.clone())
-                    }
-                },
-            })
-            .collect();
+        let codepoint_map: Vec<formatter::CodepointMap> =
+            cfg.clipboard_codepoint_map.map.list.clone();
         let opts = formatter::Options {
             emit: Format::Plain, // We'll override this below
             unwrap: true,