Reduce tap dance memory usage, move state out of data (#25415)

* Use less tap dance memory.

Use dynamically allocated sparse array for tap dance state, dynamically allocate tap dance state when needed and free it when the tap dance is done.

* new approach

* Use null, check for null

* Reformat with docker

* Use uint8 with idx rather than uint16 with keycode in state

* fix accidental change

* reformat

* Add null check

* add documentation tip suggested by tzarc

* Only allow tap dance state allocation on key down, not on key up

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

* Only allow tap dance allocation on key down, not on key up

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

* add user action required section

---------

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

docs/ChangeLog/20250831/pr25415.md

@@ -0,0 +1,47 @@
+# Tap dance state removed from `tap_dance_action_t`
+
+The tap dance state has been separated from the action structure. Custom tap dance functions now receive the state as a separate parameter instead of accessing it through `action->state`.
+
+## User Action Required
+
+If your keymap uses custom tap dance functions that access the tap dance state, you need to update your code.
+
+ - You can't use `action->state`. Instead you need to call `tap_dance_state_t *tap_dance_get_state(uint8_t tap_dance_idx)` to get the state.
+ - You now get a pointer to the state, so use `->` notation rather than `.` notation to get fields from it.
+
+### Before:
+```c
+bool process_record_user(uint16_t keycode, keyrecord_t *record) {
+    tap_dance_action_t *action;
+
+    switch (keycode) {
+        case TD(CT_CLN):
+            action = tap_dance_get(QK_TAP_DANCE_GET_INDEX(keycode));
+            if (!record->event.pressed && action->state.count && !action->state.finished) {
+                tap_dance_tap_hold_t *tap_hold = (tap_dance_tap_hold_t *)action->user_data;
+                tap_code16(tap_hold->tap);
+            }
+
+    }
+    return true;
+}
+```
+
+### After:
+```c
+bool process_record_user(uint16_t keycode, keyrecord_t *record) {
+    tap_dance_action_t *action;
+    tap_dance_state_t* state;
+
+    switch (keycode) {
+        case TD(CT_CLN):
+            action = tap_dance_get(QK_TAP_DANCE_GET_INDEX(keycode));
+            state = tap_dance_get_state(QK_TAP_DANCE_GET_INDEX(keycode));
+            if (!record->event.pressed && state != NULL && state->count && !state->finished) {
+                tap_dance_tap_hold_t *tap_hold = (tap_dance_tap_hold_t *)action->user_data;
+                tap_code16(tap_hold->tap);
+            }
+    }
+    return true;
+}
+```