Render until the output stops changing

[faresrafat3]

Fixes #588.

Escaping can depend on where word wrapping split a paragraph, so a single
render pass may produce text that the next pass formats differently. With a
small --wrap, three passes are sometimes needed, but only two were
performed, so a file mdformat had just written failed --check and a second
run changed it again:

$ printf 'A * * b\n' > test.md
$ mdformat --wrap 5 test.md && cat test.md
A *
\* b
$ mdformat --check --wrap 5 test.md
Error: File "test.md" is not formatted.

Root cause

For that input the render passes are:

pass1: 'A * *\nb\n'
pass2: 'A *\n\\* b\n'
pass3: 'A \\*\n\\* b\n'   <- fixed point

Pass 1 breaks the line after A * *, so on pass 2 the second asterisk is at
the end of a text token and loses the whitespace neighbour that had kept it
unescaped. Pass 2 then escapes it, and pass 3 escapes the first one too
because it is now adjacent to the backslash. mdformat.text() only ran two
passes, so it returned the pass-2 output, which is not a fixed point.

Changes

_api.text() now renders repeatedly until the output is unchanged, with a
bound of 9 iterations so a pathological input cannot loop forever. This makes
"what mdformat writes" and "what --check accepts" the same thing.

Verification

The reported command sequence now settles on the first run and --check
passes:

$ printf 'A * * b\n' > test.md
$ mdformat --wrap 5 test.md && cat test.md
A \*
\* b
$ mdformat --check --wrap 5 test.md
$ echo $?
0

A regression test covers exactly this (test_check__after_format_with_small_wrap).

I measured how many passes the CommonMark spec fixtures actually need, to check
that the loop is not papering over a deeper problem:

wrap	passes needed
keep	0 or 1 (no change)
no	0, 1, or 2
60	0, 1, or 2
5	0, 1, 2, or 3 (1 case)

So two passes cover everything except one case at --wrap=5, which is why the
bug only shows up at small wrap widths.

Full suite: 4283 passed, 5 skipped (4282 before, plus the new test).

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[faresrafat3]

Hi @hukkin — following up on this PR. It fixes the fixed-point rendering issue where --check would fail on freshly-formatted files. All 4283 tests pass. Let me know if you'd like any changes or if there's anything else I can help with.