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/engine/__tests__/lexer.spec.ts

@@ -56,6 +56,11 @@ describe('lexer', () => {
     expect(tokenize('"a\\"b"')[0]).toMatchObject({ type: 'STRING', value: 'a"b' });
   });
 
+  it('reads $-locked cell references as one IDENT', () => {
+    expect(types('$A$1 A$1 $A1')).toEqual(['IDENT', 'IDENT', 'IDENT', 'EOF']);
+    expect(values('$A$1 A$1 $A1')).toEqual(['$A$1', 'A$1', '$A1', '']);
+  });
+
   it('skips whitespace', () => {
     expect(types('  A1\t+\n B2 ')).toEqual(['IDENT', 'PLUS', 'IDENT', 'EOF']);
   });