Support $-locked references: relative, mixed and absolute

The engine already anticipated $A1/A$1/$A$1 in its grammar comment, but the
lexer had no rule for '$' at all -- any formula using it failed to parse.

- lexer.ts: '$' is now a valid word character, so "$A$1" tokenizes as one
  IDENT, same as any other bare word (the lexer still doesn't interpret it,
  per its "dumb lexer" design -- the parser/evaluator stay untouched, since
  evaluation is purely positional and $ has no effect on a value).
- refs.ts: parseA1()/toA1() now carry colAbs/rowAbs through the $ markers
  (backward compatible -- existing {col,row} destructuring is unaffected).
  Added cycleReferenceLock() for Excel/Numbers-style F4: relative ->
  absolute -> row-locked -> col-locked -> relative, toggling whichever
  reference sits at the caret (and the correct side of an A1:B2 range).
- document.ts: shiftFormula() now honors the lock on copy (a locked axis
  stays exactly as typed) but not on cut (a moved cell's reference follows
  it regardless of $, since cut relocates the cell itself rather than
  filling a relative offset).
- TableGrid.vue's inline cell editor and FormulaBar.vue both wire up F4 via
  the shared cycleReferenceLock() helper.
- Tests: lexer/parser coverage for $ tokenizing/parsing, a new refs.spec.ts
  for parseA1/toA1 round-tripping and the F4 cycle (incl. range side-
  picking), and document.spec.ts cases for locked-axis copy vs. cut.
- README.md / src/engine/README.md: documented the $ semantics, F4, and
  corrected a couple of now-stale claims (test count, "$ is ignored, there's
  no copy/fill", "active cell is always the last-clicked one").

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

src/components/FormulaBar.vue

@@ -1,6 +1,7 @@
 <script setup lang="ts">
-import { ref, watch } from 'vue';
+import { nextTick, ref, watch } from 'vue';
 import { useDocument } from '../store/document';
+import { cycleReferenceLock } from '../engine';
 
 const store = useDocument();
 const draft = ref('');
@@ -31,6 +32,15 @@ function onEscape() {
   draft.value = store.activeCellInput.value;
   (document.activeElement as HTMLElement)?.blur();
 }
+/** F4: cycle relative -> absolute -> row-locked -> col-locked for the cell
+ *  reference at the caret (Excel/Numbers "Bezug sperren"). */
+function onF4(e: KeyboardEvent) {
+  const el = e.target as HTMLInputElement;
+  const res = cycleReferenceLock(draft.value, el.selectionStart ?? draft.value.length);
+  if (!res) return;
+  draft.value = res.text;
+  nextTick(() => el.setSelectionRange(res.caret, res.caret));
+}
 </script>
 
 <template>
@@ -49,6 +59,7 @@ function onEscape() {
       @blur="onBlur"
       @keydown.enter.prevent="onEnter"
       @keydown.esc.prevent="onEscape"
+      @keydown.f4.prevent="onF4"
     />
   </div>
 </template>