Clear the terminal title on an empty OSC 0/2 - #623

An empty OSC 0/2 title (printf '\e]2;\a', the standard way to reset a title) was silently dropped at the native title callback: libghostty stores a cleared title as no title, getTitle returns null for it, and the callback only forwarded non-null titles. The stale title stayed in the mode line and in a title-derived buffer name until the next title report.

Changes

• src/handler.zig — titleChangedCallback forwards a cleared title as "" instead of dropping it. libghostty delivers the clear correctly end to end (empty payload still parses as change_window_title, title_changed fires unconditionally); this guard was the only drop point.
• lisp/ghostel.el — ghostel--set-title treats an empty title as "as if no title was ever set", matching Ghostty and iTerm2: ghostel--title normalizes to nil (invariant is now nil-or-non-empty, so the mode line falls back to the plain buffer name), and a title-derived buffer name reverts to ghostel--buffer-identity. The revert only applies to names title tracking has claimed and still goes through the manual-rename guard.
• ghostel--init-buffer backfills a nil ghostel--buffer-identity with the creation name so buffers created outside the ghostel command (ghostel-exec, eshell visual commands) have a revert target.
• The now-dead empty-string handling in ghostel--buffer-identification and ghostel-buffer-name-by-title is removed; the ghostel-buffer-name-function contract is now "nil or non-empty string" (custom name functions see nil on a clear, never "").

Tests

• New native PTY-matrix regression test: title set, then cleared via OSC 2+ST, OSC 2+BEL, and OSC 0+BEL — asserts nil ghostel--title and the reverted buffer name on both backends.
• New elisp unit tests: clear reverts a by-title name to the identity, declines after a manual rename, is a no-op for buffers title tracking never renamed, and leaves by-directory names alone.
• Verified live in a terminal Emacs session (bash --norc, shell integration off): OSC 0/2 set/clear round-trips update ghostel--title, the mode line (no empty-parens leftovers), and by-title renames/reverts, with manual renames respected.

Fixes #619