[222_49] fix math mode tab cycling for backtick, brackets and pipe

[1sh-repalto]

Fixes #2910
In math mode, backtick (`), square brackets ([, ]), and vertical bar (|) do not show the Tab cycle focus box popup

Summary

Enable the Tab cycle popup for keys bound to procedures (make-lprime, math-bracket-open) in math mode, so users can discover and cycle through variants using Tab.

Developer document: devel/222_49.md

Issue Found

In math-edit.scm, the tabcycle-symbols function only handled string-type keyboard bindings. Keys like `, [, and | are bound to procedures (e.g., make-lprime, math-bracket-open), so tabcycle-symbols returned an empty list and no popup was shown — even though these keys have Tab variants defined in math-kbd.scm.

;; Before: procedure bindings were rejected
(base (if (string? primary-sym)
          `((symbol-completion ,primary-sym))
          '()))  ;; <-- always empty for procedures

Changes

1. Extended lambda-to-symbol in TeXmacs/progs/math/math-edit.scm to handle make-lprime and math-bracket-open. For brackets, left and right args are concatenated for unique display (e.g., [], [[, ]], ][).

2. Modified tabcycle-symbols to use function-to-symbol for procedure-bound primary symbols:

;; After: handles both string and procedure bindings
(base (cond
        ((string? primary-sym)
         `((symbol-completion ,primary-sym)))
        ((procedure? primary-sym)
         (let ((sym (function-to-symbol kbd-res)))
           (if sym (list sym) '())))
        (else '())))

3. Fixed deduplication filter to extract primary-name from base instead of using primary-sym directly, preventing crashes when primary-sym is a procedure.

How to Test

1. Open Mogan Editor
2. Enter math mode by pressing $
3. Type ` (backtick) — verify a Tab cycle popup appears showing prime variants
4. Press Tab to cycle through the variants
5. Type [ — verify a popup appears showing bracket pair variants ([], [[, ]], ][)
6. Type | — verify a popup appears showing pipe variants
7. Type a — verify the existing Tab cycle behavior (a → α) still works correctly

tab-cycle-fix.mp4