fix(org-latex): filter-based tabularx with guaranteed registration

Root cause of \begin{tabularx}{lll} error: org-latex in this version does not
add \linewidth for tabularx environment, generating invalid LaTeX.

Fix: org-export-filter-table-functions filter that transforms rendered output:
  - \begin{tabular}{spec}     → \begin{tabularx}{\linewidth}{spec}
  - \begin{tabularx}{spec}    → \begin{tabularx}{\linewidth}{spec} (missing width fix)
  - \end{tabular}             → \end{tabularx}

No buffer modification → no recursion, works on Emacs 31.

Belt+suspenders registration: with-eval-after-load 'ox-latex + explicit
require+add-to-list in org-export-before-processing-hook ensures the filter
is always registered before the export transcoder runs.

config.el

@@ -325,19 +325,48 @@ Bound to cmd+v in org-mode and markdown-mode."
 ;;; ORG MODE — LATEX EXPORT
 ;;; ============================================================
 
-;; LaTeX table export: use tabularx automatically — no buffer modification.
-;; Setting a buffer-local variable in the hook is safe (no insert = no recursion).
-;; org-latex.el natively handles tabularx/tabulary by adding \linewidth width arg.
-(defun my/org-set-tabularx (backend)
-  "Set tabularx as default LaTeX table environment for this export."
-  (when (org-export-derived-backend-p backend 'latex)
-    (setq-local org-latex-default-table-environment "tabularx")))
-
-(add-hook 'org-export-before-processing-hook #'my/org-set-tabularx)
+;; LaTeX table export: tabular → tabularx{\linewidth}
+;; Filter on rendered LaTeX string — no buffer modification, no recursion.
+;; Handles both cases:
+;;   \begin{tabular}{lll}          → \begin{tabularx}{\linewidth}{lll}
+;;   \begin{tabularx}{lll}         → \begin{tabularx}{\linewidth}{lll}  (missing width)
+(defun my/org-latex-fix-tabularx (table _backend _info)
+  "Convert tabular → tabularx{\\linewidth} in LaTeX table transcoder output."
+  (when (stringp table)
+    ;; Fix tabularx with missing width (org versions that don't add \linewidth)
+    (setq table
+          (replace-regexp-in-string
+           "\\\\begin{tabularx}{\\([^\\\\][^}]*\\)}"
+           "\\\\begin{tabularx}{\\\\linewidth}{\\1}"
+           table))
+    ;; Convert plain tabular to tabularx
+    (setq table
+          (replace-regexp-in-string
+           "\\\\begin{tabular}{\\([^}]*\\)}"
+           "\\\\begin{tabularx}{\\\\linewidth}{\\1}"
+           table))
+    (setq table
+          (replace-regexp-in-string
+           "\\\\end{tabular}"
+           "\\\\end{tabularx}"
+           table)))
+  table)
 
+;; Register filter on ox-latex load AND ensure it via a pre-processing hook
+;; (belt+suspenders: whichever fires first wins, both are idempotent).
 (with-eval-after-load 'ox-latex
+  (add-to-list 'org-export-filter-table-functions #'my/org-latex-fix-tabularx)
   (add-to-list 'org-latex-packages-alist '("" "tabularx")))
 
+(defun my/org-ensure-tabularx-filter (backend)
+  "Force ox-latex load and register tabularx filter before LaTeX export."
+  (when (org-export-derived-backend-p backend 'latex)
+    (require 'ox-latex)
+    (add-to-list 'org-export-filter-table-functions #'my/org-latex-fix-tabularx)
+    (add-to-list 'org-latex-packages-alist '("" "tabularx"))))
+
+(add-hook 'org-export-before-processing-hook #'my/org-ensure-tabularx-filter)
+
 ;; Optional: enable booktabs style (horizontal rules in tables)
 ;; (setq org-latex-tables-booktabs t)