Darts: a second aiming control with two sweeping lines

[margani]

Closes #372

What changed (www/games/darts/index.html, tools/test.js)

• Engine: sweep(u), an even back-and-forth from -1 to 1.
• Page: a new control. Tap (or Space/Enter) stops an upright line, then a level line; the dart goes where they cross, with the same SCATTER as before. A badge above the board switches between Drag and Two lines at once, mid-turn too; it is saved in prefs.aim (default drag, so nobody's game changes). The daily shows it as well.
• Strings in five languages, a help paragraph, and the keyboard row.

Proof

• node tools/test.js: 4211 checks green. With main's darts/index.html and the new tests: 6 failures (sweep, reach, saved switch, tap/Space path, switch in the daily, and the check count).
• tools/browser-check.sh: 36 of 36 pages.
• Playwright on a local server, 9 of 9 checks, no console errors: Persian status per stage, two touch taps throw one dart, Space then Enter throws one, the switch goes both ways and is saved, the daily has it. Screenshots at 390 px light/dark Persian and 1280 px dark English, plus the help window. The first screenshots showed the lines running past the round board into the canvas corners; they are now clipped to the board.

Not tested

• On a real phone, so whether 1.3 s per sweep feels right is a guess from the timing maths in the commit message, not playtesting.

[margani]

Diff review: the new control lives entirely beside the old one. barsOn() reuses canAim(), so lines never run during a bot turn, a flight, a dialog or a finished game, and fly() clears bars so the next dart starts fresh. Pointer and keyboard handlers return early in two-line mode, so the drag path is untouched when the switch is on Drag (the default). Pause clears the lines. Not covered by tests: the feel of the sweep speed on a phone.