fix: physical keyboard handling in mnemonic entry + trackball sensitivity

Mnemonic word entry only understood the on-screen keyboard:
- letters: the screen posts uppercase A-Z, but the T-Deck keyboard sends
  lowercase ascii, so typed letters were ignored; accept either case
- enter: the on-screen enter button is inactive during mnemonic entry,
  but a physical keyboard can post BTN_KEYBOARD_ENTER at any time, which
  set done_entering_words early and tripped the words_entered==nwords
  assert (mnemonic.c:865); only honour enter when the on-screen button
  would be active
- letter validity: the on-screen keyboard disables letters that match no
  word, a physical keyboard cannot - reject letters yielding a stem with
  no matching words (previously would trip the possible_words>0 assert)

Also halve trackball sensitivity: require 2 edges per scroll step, with
the accumulator reset on direction change or a 300ms pause.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: 3rdIteration

main/input/tdeck.inc

@@ -23,6 +23,12 @@
 // caps the overall scroll rate
 #define TB_DEBOUNCE_US      2000
 
+// Edges required per scroll step - raise to reduce sensitivity
+#define TB_EDGES_PER_EVENT  2
+
+// Edges further apart than this do not accumulate towards a step
+#define TB_GESTURE_TIMEOUT_US 300000
+
 static void tdeck_keyboard_task(void* unused)
 {
     gpio_set_direction(BOARD_POWERON_GPIO, GPIO_MODE_OUTPUT);
@@ -79,13 +85,26 @@ static QueueHandle_t tb_queue = NULL;
 static void tb_isr(void* arg)
 {
     static int64_t last_event_us = 0;
+    static tb_nav_t last_nav = TB_NAV_PREV;
+    static uint8_t edge_count = 0;
+
     const int64_t now = esp_timer_get_time();
-    if (now - last_event_us < TB_DEBOUNCE_US) {
+    const int64_t elapsed = now - last_event_us;
+    if (elapsed < TB_DEBOUNCE_US) {
         return;
     }
     last_event_us = now;
 
     const tb_nav_t nav = (tb_nav_t)(uintptr_t)arg;
+    if (nav != last_nav || elapsed > TB_GESTURE_TIMEOUT_US) {
+        last_nav = nav;
+        edge_count = 0;
+    }
+    if (++edge_count < TB_EDGES_PER_EVENT) {
+        return;
+    }
+    edge_count = 0;
+
     BaseType_t higher_priority_woken = pdFALSE;
     xQueueSendFromISR(tb_queue, &nav, &higher_priority_woken);
     if (higher_priority_woken == pdTRUE) {

main/process/mnemonic.c

@@ -818,13 +818,24 @@ static size_t get_wordlist_words(
                 // Wait for kb button click
                 gui_activity_wait_event(enter_word_activity, GUI_BUTTON_EVENT, ESP_EVENT_ANY_ID, NULL, &ev_id, NULL, 0);
                 selected_backspace = (ev_id == BTN_KEYBOARD_BACKSPACE);
-                done_entering_words = (ev_id == BTN_KEYBOARD_ENTER);
-                if (!selected_backspace && !done_entering_words) {
-                    // Character/letter was clicked
-                    const char letter_selected = ev_id - BTN_KEYBOARD_ASCII_OFFSET;
+                // A physical keyboard can send 'enter' at any time - only honour it
+                // where the on-screen enter button would be active (see enable_relevant_chars)
+                done_entering_words = (ev_id == BTN_KEYBOARD_ENTER && !is_mnemonic && !char_index);
+                if (!selected_backspace && ev_id >= BTN_KEYBOARD_ASCII_OFFSET) {
+                    // Character/letter was clicked/typed (physical keyboards send lowercase)
+                    const char letter_selected = toupper((int)(ev_id - BTN_KEYBOARD_ASCII_OFFSET));
                     if (letter_selected >= 'A' && letter_selected <= 'Z') {
+                        // Only accept a letter if some word still matches the new stem -
+                        // the on-screen keyboard disables impossible letters, but a
+                        // physical keyboard cannot
                         word[char_index] = tolower(letter_selected);
-                        word[++char_index] = '\0';
+                        word[char_index + 1] = '\0';
+                        if (valid_words(word, char_index + 1, p_filter_words, num_filter_words, possible_word_list,
+                                NUM_WORDS_SELECT, &exact_match)) {
+                            ++char_index;
+                        } else {
+                            word[char_index] = '\0';
+                        }
                     }
                 }
             }