encoder tests: align mock 'index' field with the API's uint8_t

Apple clang 21 fails the encoder test builds with errors like:

    quantum/encoder/tests/encoder_tests.cpp:37:40: error:
      non-constant-expression cannot be narrowed from type 'uint8_t'
      (aka 'unsigned char') to 'int8_t' (aka 'signed char') in
      initializer list [-Wc++11-narrowing]
       37 |     updates[updates_array_idx % 32] = {index, clockwise};
          |                                        ^~~~~

`encoder_update_kb()` takes `uint8_t index`, but the test-only
`struct update` declared the captured field as `int8_t`. The
aggregate-init `{index, clockwise}` then narrows uint8_t -> int8_t,
which clang's stricter -Wc++11-narrowing rejects (gcc was lenient
about it, which is why Linux CI didn't catch this).

The signed type was a mismatch, not a deliberate choice — encoder
indices are non-negative — so widen the field to `uint8_t` to match
the real API instead of papering over the narrowing with a cast.
Applied identically to all six encoder test variants.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: scottlamb

quantum/encoder/tests/encoder_tests.cpp

@@ -26,8 +26,8 @@ extern "C" {
 }
 
 struct update {
-    int8_t index;
-    bool   clockwise;
+    uint8_t index;
+    bool    clockwise;
 };
 
 uint8_t updates_array_idx = 0;

quantum/encoder/tests/encoder_tests_split_left_eq_right.cpp

@@ -26,8 +26,8 @@ extern "C" {
 }
 
 struct update {
-    int8_t index;
-    bool   clockwise;
+    uint8_t index;
+    bool    clockwise;
 };
 
 uint8_t updates_array_idx = 0;

quantum/encoder/tests/encoder_tests_split_left_gt_right.cpp

@@ -26,8 +26,8 @@ extern "C" {
 }
 
 struct update {
-    int8_t index;
-    bool   clockwise;
+    uint8_t index;
+    bool    clockwise;
 };
 
 uint8_t updates_array_idx = 0;

quantum/encoder/tests/encoder_tests_split_left_lt_right.cpp

@@ -26,8 +26,8 @@ extern "C" {
 }
 
 struct update {
-    int8_t index;
-    bool   clockwise;
+    uint8_t index;
+    bool    clockwise;
 };
 
 uint8_t updates_array_idx = 0;

quantum/encoder/tests/encoder_tests_split_no_left.cpp

@@ -26,8 +26,8 @@ extern "C" {
 }
 
 struct update {
-    int8_t index;
-    bool   clockwise;
+    uint8_t index;
+    bool    clockwise;
 };
 
 uint8_t updates_array_idx = 0;

quantum/encoder/tests/encoder_tests_split_no_right.cpp

@@ -26,8 +26,8 @@ extern "C" {
 }
 
 struct update {
-    int8_t index;
-    bool   clockwise;
+    uint8_t index;
+    bool    clockwise;
 };
 
 uint8_t updates_array_idx = 0;