fix(stats): count mined cards once and keep lexical stats exact after deletes

- Card creation now owns mined-card counting; remove the outer single and
  multi-line mining increments that double counted each card.
- Preserve fractional first_seen/last_seen precision when session deletes
  compare and refresh lexical aggregates.
- Derive vocabulary_visible from the stored word row on every write so the
  new-word charts agree with the filtered vocabulary summary.
- Bump the lexical rollup version so existing databases re-derive visibility
  and repair stale last_seen values once in the background worker.

src/main/runtime/mining-actions.test.ts

@@ -53,7 +53,6 @@ test('mine sentence digit handler forwards all dependencies', () => {
     getCurrentSecondarySubText: () => 'secondary',
     showMpvOsd: (text) => calls.push(`osd:${text}`),
     logError: (message) => calls.push(`err:${message}`),
-    onCardsMined: (count) => calls.push(`cards:${count}`),
     handleMineSentenceDigitCore: (count, options) => {
       assert.equal(count, 4);
       assert.equal(options.subtitleTimingTracker, tracker);
@@ -61,10 +60,9 @@ test('mine sentence digit handler forwards all dependencies', () => {
       assert.equal(options.getCurrentSecondarySubText(), 'secondary');
       options.showMpvOsd('mine');
       options.logError('boom', new Error('x'));
-      options.onCardsMined(2);
     },
   });
 
   handleMineSentenceDigit(4);
-  assert.deepEqual(calls, ['osd:mine', 'err:boom', 'cards:2']);
+  assert.deepEqual(calls, ['osd:mine', 'err:boom']);
 });