test+fix: parity-line e2e coverage, and fix #4 built-ins unreachable via REPL

[DDecoene]

Brings Playwright e2e coverage up to date for everything shipped since the last e2e (MODIFY STRUCTURE, 2026-06-24), and fixes a real bug the e2e surfaced.

The bug (fixed)

The #4 built-ins (ROUND, MOD, MAX, MIN, TIME, YEAR, MONTH, DAY — @kas2804's PR #17, shipped in 0.8.0) were implemented in Builtins.ts but never registered in the parser's BUILTIN_FUNCTIONS whitelist. So ? ROUND(3.14159, 2) failed with Unknown command: ( — the functions were unreachable from the REPL. The vitest unit tests passed only because they call callStateless directly, bypassing the parser. This is exactly the unit-vs-e2e gap that motivated this work. Fixed by registering the eight names; added tests/BuiltinsParse.test.ts (8 cases) that drive the functions through the parser as a regression guard.

New e2e — tests/parity-commands.spec.ts (4 tests)

• ? / ?? print (Implement the ? command — print an expression #2)
• More built-in functions: ROUND(), MOD(), MAX(), MIN(), TIME(), YEAR()/MONTH()/DAY() #4 built-ins, end-to-end through the REPL (now that they work)
• SUM / AVERAGE (Add SUM and AVERAGE commands #3)
• SORT ON … TO (SORT TO — physically sorted copy of a table #8)

Process fix

CLAUDE.md Definition of Done now requires a Playwright e2e per user-facing command, not just a unit test — the root cause of this drift.

Verification

Full vitest 224 passing, tsc clean; the 4 new e2e tests pass live against a real browser (re-ran independently).

Targets release/v1.0.0 per GitFlow.