Preserve Ghostel anchoring across font scaling

[emil-e]

Summary

• add shared helpers for finding visible Ghostel windows and anchored windows
• force size adjustment after leaving copy/read-only mode
• preserve bottom anchoring when text/global font scaling changes terminal geometry
• add a small agent-output script for manual anchoring tests

Verification

• emacs --batch -Q -L lisp --eval "(setq byte-compile-error-on-warn t)" -f batch-byte-compile lisp/ghostel.el
• emacs --batch -Q -L lisp -L test -l ert -l test/ghostel-test-helpers.el -l test/ghostel-modes-test.el --eval "(ert-run-tests-batch-and-exit '(not (tag native)))"
• emacs --batch -Q -L lisp -L test -l ert -l test/ghostel-test-helpers.el -l test/ghostel-terminal-test.el --eval "(ert-run-tests-batch-and-exit '(not (tag native)))"
• emacs --batch -Q -L lisp -L test -l ert -l test/ghostel-test-helpers.el -l test/ghostel-scroll-test.el --eval "(ert-run-tests-batch-and-exit '(not (tag native)))"
• checkdoc on lisp/ghostel.el
• bash -n tools/agent-output.sh
• smoke run of tools/agent-output.sh
• git diff --check