DATEADD() built-in — date arithmetic

[DDecoene]

Why

W3Script has no date arithmetic. CTOD/DTOC only reformat, and YEAR/MONTH/DAY only decompose. There is no way to say "the day after this one", which demos/overtime.prg (#46) needs 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.

Scope

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

Follow the WEEK() precedent from #44 exactly:

• Computed in UTC (Date.UTC + getUTC*) so a local timezone offset can never shift the day.
• Accepts ISO YYYY-MM-DD or MM/DD/YY; composes with CTOD().
• ISO-shaped but impossible dates (2024-02-30, 2023-02-29) return '' rather than silently rolling over, matching WEEK()'s round-trip rejection.
• Invalid input returns ''.
• Crosses month, year and leap-day boundaries correctly.

Must be registered in BUILTIN_FUNCTIONS in src/interpreter/Parser.ts. Implementing it in Builtins.ts alone leaves it unreachable (Unknown command: () — that's how the #4 built-ins shipped broken.

Tests

Per CLAUDE.md → Test discipline:

• tests/Builtins.test.ts (direct) and tests/BuiltinsParse.test.ts (through the parser) — both are required.
• A Playwright case typing ? DATEADD(...) into the REPL and asserting the printed value.
• Cover: same month, month rollover, year rollover, leap day (2024-02-28 → 2024-02-29), non-leap (2023-02-28 → 2023-03-01), negative n, n = 0, invalid input.

Docs

Built-ins tables in README.md and CLAUDE.md; CHANGELOG.md under v1.2.0.

Assistant parity

Not required — built-ins are expression-level functions used inside commands, not GUI-shaped actions, and none of them (ROUND, WEEK, YEAR, …) have sidebar entries. Note this in the PR.

Branch

feature/<name> off release/v1.2.0, PR targets release/v1.2.0. Blocks #46.

[DDecoene]

Merged in #53 into release/v1.2.0.