fix: Round-trip block scalars with leading space on first line

[Noethix55555]

Fixes #692.

Problem

stringify() of a multiline string whose first content line starts with a space produced a block scalar whose declared indentation indicator did not match the body indentation. At the document root ctx.indent is '', but the indicator was emitted as 1, while body lines were prefixed with the (empty) root indent. As a result the output either silently dropped a leading space per line or was not parseable by parse():

parse(stringify('  a\n  b'))           // => " a\n b"  (lost a leading space)
parse(stringify('  indented\nlines'))  // throws YAMLParseError

Fix

In blockString (src/stringify/stringifyString.ts) introduce a bodyIndent that honors the declared indicator at the root (a single space when the value starts with a space and indent is empty), and use it for the body in both the literal and folded paths. The folded path additionally accounts for this added indent when deciding whether the first line is "more-indented", so a more-indented first line is no longer incorrectly folded.

Non-root and non-leading-space behavior is unchanged.

Tests

• Added round-trip regression tests in tests/doc/stringify.ts covering several leading-space cases. These fail on main and pass with the fix.
• Updated the existing tests/doc/foldFlowLines.ts "More-indented first line (Long strings that start with spaces aren't rendered properly (and lose newlines) #55)" test: it was asserting the buggy output, whose expected string was itself unparseable. It now asserts the corrected, round-trippable output and additionally checks that the output parses back to the original.

The full vitest run suite passes (the two external test-suite files require submodule data and fail identically on a pristine tree).

LLM assistance

This pull request was prepared with the assistance of an LLM (declared per CONTRIBUTING.md). All changes were reviewed and verified locally.