Make evil-ghostel ESC routing discoverable

[dakra]

Closes #539

New evil/Doom users starting a fullscreen TUI (e.g. Claude Code with /tui fullscreen) felt trapped: with the default auto routing, alt-screen ESC goes to the app, so normal state is unreachable via ESC, the leader key is dead in insert state, and nothing pointed at the escape hatch.

Discoverability (febcb6d)

• One-time hint: the first time auto routing sends ESC to an alt-screen app, the echo area shows ESC sent to the terminal app (alt-screen); C-c C-r routes it to evil instead (once per session).
• C-c C-r is bound to evil-ghostel-toggle-send-escape, now a two-state toggle: from auto it switches to whichever mode differs from auto's current effect (evil while an alt-screen app runs, terminal otherwise); from an explicit mode back to auto. Every press visibly changes the routing — previously, cycling auto → terminal in an alt-screen app changed nothing observable, which is exactly what confused users in Need more visibility on evil-ghostel-toggle-send-esc #539. Numeric prefixes 1/2/3 still set a mode directly, and landing on auto messages the effective routing (auto (now → terminal)).
• C-c <escape> runs evil-force-normal-state as a one-shot switch that leaves the routing untouched. It's bound on the <escape> function key, so it works on GUI frames and on ttys speaking the kitty keyboard protocol (via kkp.el); on legacy ttys the event never occurs, so it can't shadow the C-c M-… bindings.
• Docs: new ESC routing section in the manual, a note in the main README, a new extensions/evil-ghostel/README.md, and an expanded package commentary.

Alt-screen detection cleanup (d206461)

Three different alt-screen checks existed: the native ghostel--alt-screen-p (reads the active screen), line-mode's 1049-or-1047 mode query, and evil-ghostel's 1049-only query. The mode queries miss alt screens entered via other DEC modes — e.g. auto ESC routing kept ESC in evil for a TUI using mode 1047.

A new public ghostel-alt-screen-p wraps the native check and is now used for line-mode pause/resume and all evil-ghostel alt-screen decisions; tests stub the native function instead of ghostel--mode-enabled. The extension .elc Make rule also gained a dependency on the core .elc files so parallel builds can't compile evil-ghostel against a stale ghostel.elc.

Testing

• make -j8 all (build, elisp + native tests, lint) and make test-evil (130/130) pass.
• Live-verified in real Emacs sessions (tty and GUI): the trapped-user journey from Need more visibility on evil-ghostel-toggle-send-esc #539, the hint, the toggle in and out of alt-screen, C-c <escape> on a GUI frame, and the mode-1047/47 coverage of the new predicate (where the old flag check provably returned nil).