smf: improve sibling transitions speed

Sibling transitions are now detected in smf_set_state() when source and
destination share the same parent, removing the need for states to flag
the sibling transition explicitly.

Restore include/zephyr/smf.h and lib/smf/Kconfig to match main so this
change only carries the functional update in lib/smf/smf.c, and add a
micro-benchmark test. For accurate statistics, run the test on a board.

Micro-benchmark setup
- Board: NUCLEO-F746ZG (STM32F746, ~192 MHz)
- 200000 iterations per state transition
- Simple HSM (letters are nodes; R is root):

=== SMF transition micro-benchmark ===
        R
      /   \
     A     B
    / \   / \
   C   D E   F
  / \         \
 G   H         J

Current smf_set_state:
======================
Sibling Transitions
(A->B) : 459 cycles/transition (2390 ns)
(C->D) : 482 cycles/transition (2510 ns)
(G->H) : 522 cycles/transition (2718 ns)

Other Transitions
(G<->G) : 237 cycles/transition (1234 ns)
(C->G) : 343 cycles/transition (1786 ns)
(A->H) : 452 cycles/transition (2354 ns)
(G->D) : 651 cycles/transition (3390 ns)
(D->E) : 752 cycles/transition (3916 ns)
(J->D) : 893 cycles/transition (4651 ns)
(J->G) : 1077 cycles/transition (5609 ns)

New smf_set_state:
==================
Sibling Transitions
(A->B) : 356 cycles/transition (1854 ns)(22% faster)
(C->D) : 356 cycles/transition (1854 ns)(26% faster)
(G->H) : 356 cycles/transition (1854 ns)(32% faster)

Other Transitions
(G<->G) : 258 cycles/transition (1343 ns)(9% slower)
(C->G) : 356 cycles/transition (1854 ns)(4% slower)
(A->H) : 464 cycles/transition (2416 ns)(3% slower)
(G->D) : 707 cycles/transition (3682 ns)(9% slower)
(D->E) : 797 cycles/transition (4151 ns)(6% slower)
(J->D) : 970 cycles/transition (5052 ns)(9% slower)
(J->G) : 1157 cycles/transition (6026 ns)(8% slower)

This change makes sibling transitions
deterministic and cheaper (356 cycles on nsim),
at the cost of a small increase for deeper/LCA
transitions (~7–9%). This is a net win for state machines
that mostly hop between siblings.

Signed-off-by: Vladislav Kulikov <vlad_kulikov_c@pm.me>

lib/smf/smf.c

@@ -300,7 +300,10 @@ void smf_set_state(struct smf_ctx *const ctx, const struct smf_state *new_state)
 #ifdef CONFIG_SMF_ANCESTOR_SUPPORT
 	const struct smf_state *topmost;
 
-	if (is_descendant_of(ctx->executing, new_state)) {
+	if (ctx->executing != new_state && ctx->executing->parent == new_state->parent) {
+		/* Optimize sibling transitions (different states under same parent) */
+		topmost = ctx->executing->parent;
+	} else if (is_descendant_of(ctx->executing, new_state)) {
 		/* new state is a parent of where we are now*/
 		topmost = new_state;
 	} else if (is_descendant_of(new_state, ctx->executing)) {