refactor(tp): decouple retained core from dash

Replace the two Dash list helpers in tp-core with cl-lib primitives so tp-surface can load through its minimal public module chain. The legacy facade continues to declare Dash for layer and stack APIs.

Verified: standalone tp-surface load without Dash, 718 ERT tests, WERROR compile-all, and git diff --check.

tp-core.el

@@ -21,7 +21,6 @@
 ;;; Code:
 
 (require 'cl-lib)
-(require 'dash)
 (require 'seq)
 (require 'text-property-search)
 
@@ -866,8 +865,9 @@ Returns the list of FUNCTION's non-nil results, in interval order
              (interval-end (nth 1 tp))
              (interval-props (nth 2 tp))
              (top-props
-              (if-let ((idx (-elem-index 'tp-layers interval-props)))
-                  (-remove-at-indices (list idx (1+ idx)) interval-props)
+              (if-let ((idx (cl-position 'tp-layers interval-props)))
+                  (append (cl-subseq interval-props 0 idx)
+                          (nthcdr (+ idx 2) interval-props))
                 interval-props))
              (below-props-lst (plist-get interval-props 'tp-layers)))
         (funcall function