Fix #1452: Piped sync child leaves colorizeStderr mid-line after the send confirm, so the next diagnostic line loses its severity colour

[philcunliffe]

Feature or issue

Since PR #1448 the wizard's sync step spawns hyp sync with a piped stderr, which turns off the child's own painting (useColor is false on a pipe), leaving the parent's colorizeStderr as the only painter. That wrap infers the cursor from its own writes, but the child's send confirm ends without a newline and the answer, plus the newline the tty echoes beside it, never passes through the wrap. The wrap therefore still believes it is mid-question when the child's next diagnostic arrives, and paintChunk's line-start gate leaves it plain: hyp sync: nothing was sent - the sink driver is holding every tick reached the terminal red under stdio: 'inherit' and unpainted afterwards. Reverting only the src/ half of this branch reproduces it exactly, with the new wizard test asserting the red hyp sync: prefix and receiving the bare text.

Solution

• colorizeStderr gains a symbol-keyed hook and style.js exports resyncLineStart(stream), which tells a wrap the terminal is back at a line start. It is a no-op on an unwrapped stream, which is every stream that was not painting anyway.
• The runSyncChild echo loop tracks whether the chunk it last relayed ended without a newline and resyncs before writing the next one, so the diagnostic that follows an answered confirm is classified again.
• Verified fail-then-pass: with the two src/ files reverted to their pre-fix content the new wizard test fails on the missing SGR prefix and the style test file fails to load; with the fix restored all 42 tests in the two files pass. Full npm test and npm run typecheck show no new failures (the two hyparquet-pin test failures and the squirreling type error are present on master without this branch).

Code: +37 / -5 lines

Fixes #1452