fix(column): keep track of number of lines with number of signs

Problem:  Some edge cases to the old (pre-#26406) and current "b_signcols"
          structure result in an incorrectly sized "auto" 'signcolumn'.
Solution: * Implement a simpler 'signcolumn' validation strategy by immediately
            counting the number of signs in a range upon sign insertion and
            deletion. Decrease in performance here but there is a clear path
            forward to decreasing this performance hit by moving signs to a
            dedicated marktree, or by adding meta-data to the existing
            marktree which may be queried more efficiently?
          * Also replace "max_count" and keep track of the number of lines with
            a certain number of signs. This makes it so that it is no longer
            necessary to scan the entire buffer when the maximum number of signs
            decreases. This likely makes the commit a net increase in performance.
          * To ensure correctness we also have re-initialize the count for an
            edited region that spans multiple lines. Such an edit may move the
            signs within it. Thus we count and decrement before splicing the
            marktree and count and increment after.

src/nvim/buffer_defs.h

@@ -685,10 +685,12 @@ struct file_buffer {
                                 // may use a different synblock_T.
 
   struct {
-    int max;                         // maximum number of signs on a single line
-    int max_count;                   // number of lines with max number of signs
-    bool resized;                    // whether max changed at start of redraw
-    Map(int, SignRange) invalid[1];  // map of invalid ranges to be checked
+    int max;                    // maximum number of signs on a single line
+    int count[SIGN_SHOW_MAX];   // number of lines with number of signs
+    bool resized;               // whether max changed at start of redraw
+    bool autom;                 // whether 'signcolumn' is displayed in "auto:n>1"
+                                // configured window. "b_signcols" calculation
+                                // is skipped if false.
   } b_signcols;
 
   Terminal *terminal;           // Terminal instance associated with the buffer