What

Makes timed tests (-s) a true Monkeytype-style time attack: instead of ending the moment you reach the end of a fixed prompt, fresh words stream in as you type and the test ends only when the clock hits zero.

Closes #6.

Why

Today, thokr -s 60 over the default 15 words is over in a couple of seconds — the timer is really just a cap, not a duration. A timed test people actually want runs for the full time and keeps feeding you words. #6 has asked for this ("continually load new words to meet time limit / scroll lines") since early 2023.

How

• thok: a refill_words pool; when it's non-empty, refill_if_needed() tops the prompt up whenever the runway ahead of the cursor runs low, and has_finished() ends the test on the timer only — never by reaching the end of the words.
• main: seeds the pool from the active language, but only in the default word mode with -s. A custom prompt (-p) or sentences (-f) are finite text, so they still end at their own length.
• ui: because the prompt is now unbounded, the running view scrolls a fixed 3-line window (anchored to the cursor line) instead of expanding to fit — the layout stays centered and stable. The hardware-cursor math tracks the same window so it can't drift from the rendered text.

Verification

• cargo build, cargo test, cargo clippy, cargo fmt --check all clean; new unit tests cover refill-keeps-runway, timer-only finish, and that non-continuous mode still finishes by length.
• Manually driven through a PTY: the timer counts down, the prompt streams well past the initial buffer, and the test ends on the clock with a normal results screen.

Backwards compatible: non--s runs and -p/-f are unchanged.