vim-patch:9.1.1435: completion: various flaws in fuzzy completion (#34335)

Problem:  completion: various flaws in fuzzy completion
Solution: fix the issues (Girish Palya)

- Remove the brittle `qsort()` on `compl_match_array`.
- Add a stable, non-recursive `mergesort` for the internal doubly
  linked list of matches.
- The sort now happens directly on the internal representation (`compl_T`),
  preserving sync with external structures and making sorting stable.
- Update fuzzy match logic to enforce `max_matches` limits after
  sorting.
- Remove `trim_compl_match_array()`, which is no longer necessary.
- Fixe test failures by correctly setting `selected` index and
  maintaining match consistency.
- Introduce `mergesort_list()` in `misc2.c`, which operates generically
  over doubly linked lists.
- Remove `pum_score` and `pum_idx` variables

fixes: vim/vim#17387
closes: vim/vim#17430

8cd42a58b4

Co-authored-by: Girish Palya <girishji@gmail.com>

src/nvim/memory.h

@@ -21,6 +21,10 @@ typedef void *(*MemCalloc)(size_t, size_t);
 /// `realloc()` function signature
 typedef void *(*MemRealloc)(void *, size_t);
 
+typedef void *(*MergeSortGetFunc)(void *);
+typedef void (*MergeSortSetFunc)(void *, void *);
+typedef int (*MergeSortCompareFunc)(const void *, const void *);
+
 #ifdef UNIT_TESTING
 /// When unit testing: pointer to the `malloc()` function, may be altered
 extern MemMalloc mem_malloc;