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/lexer.ts

@@ -53,7 +53,11 @@ function isDigit(ch: string): boolean {
   return ch >= '0' && ch <= '9';
 }
 function isWordStart(ch: string): boolean {
-  return (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z') || ch === '_';
+  // '$' only ever appears inside a cell reference ("$A1", "A$1", "$A$1"), but
+  // the lexer doesn't distinguish those from any other bare word (see the
+  // module doc) -- it's just part of the word here, and the parser is the one
+  // that validates/interprets it as a locked reference axis.
+  return (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z') || ch === '_' || ch === '$';
 }
 function isWordPart(ch: string): boolean {
   return isWordStart(ch) || isDigit(ch);