Keep a wrapped line in the paragraph it belongs to

[ptheofan]

A pushed document came back with every paragraph styled as a heading —
bold, oversized, no bullets, no monospace. #60 clears inherited styles,
but the paragraphs never reach the code that would clear them.

Root cause. A soft-wrapped source line became a literal newline in
the model. insertText does not read that as a line break: it ends the
paragraph there and starts another, so one model element becomes two Doc
paragraphs. buildFormattingFromApiDoc matches by exact text, so neither
half matches, apiIdx stops advancing, and with a lookahead window of 10
every element after it is skipped. The text then keeps whatever
insertText gave it — the style of the character before the insertion
point, which after a heading is a heading.

Measured, not inferred: the stored .remote.md from a real push has
every paragraph prefixed ###, and the log shows only 24 formatting
requests emitted for the whole document.

Fix. A soft break renders as a space in markdown, so emit a space. A
hard break is a line break within the paragraph, which Docs spells as a
vertical tab (U+000B); emit that so the paragraph stays whole, and
reverse-convert it back to a markdown hard break so it does not read as a
difference on every later sync.

Verification

Three converter tests plus one round-trip test, all watched failing
first — including never leaves a newline in an inline element, which
covers headings, list items and body text together.

One trap worth recording: importing the constant from
MarkdownToDocsConverter into DocsToMarkdownConverter created a
circular import that silently left it undefined and broke 23 unrelated
tests. It now lives in @shared/constants.

pnpm test 1039 passed / 62 files · typecheck clean · lint clean.

Closes #62

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