feat: DATEADD() date-arithmetic built-in (#52)

[DDecoene]

Closes #52. Unblocks #46.

Why

W3Script has no date arithmetic. CTOD/DTOC only reformat, YEAR/MONTH/DAY only decompose — nothing can say "the day after this one". demos/overtime.prg needs it to derive TIMESHEET.WORKDATE from the week's Monday (WEEKDATE + (DOW-1) days). Surfaced while designing #46; the demo should have nothing to work around.

What

DATEADD(date, n) → the ISO date n days later (n may be negative), as YYYY-MM-DD.

• Computed in UTC, so month/year/leap-day boundaries are exact and no local timezone offset can shift the day. (Day arithmetic in local time would also have to worry about DST.)
• Impossible ISO dates (2023-02-29, 2024-13-01) return '' rather than silently rolling over, matching WEEK()'s round-trip rejection.
• Accepts ISO YYYY-MM-DD or MM/DD/YY; composes with CTOD().
• WEEK() already parsed dates in exactly this way, so that logic is extracted into a shared parseDateUTC() instead of duplicated. No behavior change to WEEK() (its tests are untouched and still pass).
• Registered in BUILTIN_FUNCTIONS in Parser.ts — omitting that is how the More built-in functions: ROUND(), MOD(), MAX(), MIN(), TIME(), YEAR()/MONTH()/DAY() #4 built-ins shipped unreachable.

Assistant parity

Not surfaced, consistent with every other built-in (ROUND, WEEK, YEAR, …): built-ins are expression-level functions used inside commands, not GUI-shaped actions, and nothing in the sidebar or HELP enumerates them.

Test plan

Suites run serially (they share data/).

• npx tsc --noEmit clean.
• npm test — 369/369 vitest (was 358). New cases in tests/Builtins.test.ts (direct: same month, month/year rollover, leap day 2024-02-28→2024-02-29, non-leap 2023-02-28→2023-03-01, negative n, n = 0 normalising MM/DD/YY→ISO, Monday+4, invalid/impossible → '') and tests/BuiltinsParse.test.ts (through the parser, plus composing with CTOD).
• npx playwright test — 84/84 (was 83). New parity-commands.spec.ts case types ? DATEADD(...) into the real REPL and asserts the printed value.

Docs

CHANGELOG.md, README.md and CLAUDE.md built-ins tables, test counts.