[MFA-G]

Summary

dump() wrote a root-level block mapping key that starts with --- or
... as a plain scalar at column 0, where those prefixes are document
boundary markers. The result either fails to reload or reloads with the key
silently changed:

dump({ '... x': 1 })   // "... x: 1\n"   -> load -> { x: 1 }
dump({ '--- x': 1 })   // "--- x: 1\n"   -> load throws

Cause

scalarLayout() derives shiftOfFirstLine from the scalar's level.
writeBlockMapping() passes level + 1 for a key, because the key scalar is
one level deeper in the AST, but writes the key on the mapping's own line. At
the document root the key really starts at column 0 while the layout reports
indent * 1 = 2, and canUsePlain() only applies the c-forbidden guard when
shiftOfFirstLine === 0. The guard therefore never fired for keys.

For a block-context key, report the mapping's indentation
(indent * (level - 1)) as the first-line shift. Flow keys keep the old
value: they always follow { or , and cannot start a line.

Result

dump({ '... x': 1 })                  // "'... x': 1\n"
dump({ '--- x': 1 })                  // "'--- x': 1\n"
dump({ a: { '... x': 1 } })           // "a:\n  ... x: 1\n"   (unchanged)
dump([{ '... x': 1 }])                // "- ... x: 1\n"       (unchanged)

Only content that is actually written at column 0 is quoted, so nested and
sequence-item keys stay plain. The diff-summary example from the report
(--- a/src/gateway/server.ts, ... 2 files omitted) now round-trips too.

Tests

Added quotes a root block mapping key that would read as a document marker
to test/core/units/dump-scalar-styles.test.mjs; it asserts the dumped text
and the reloaded value for --- x, ... x, ---, ..., indent: 4, and
that nested and sequence-item keys stay plain. The new test fails on the
current master and passes with the change.

npm test (lint, build, tsc --noEmit, yaml-test-suite, core tests):
1647 tests, 1634 pass, 0 fail, 13 skipped.

Fixes #805.