In a flow sequence or mapping a comma separates entries, so one must
precede it. The parser advanced past a comma without checking that an
entry came before, accepting a leading comma ([ , a ]) or a repeated comma
([ a, , b ]) as empty entries. Each flow loop now tracks whether an entry
precedes the comma and rejects it otherwise. A single trailing comma
([ a, b, ]) and empty collections ([], {}) remain valid, since the closing
bracket is handled before that check.
Measured against the official YAML test-suite, parse-level conformance
rises from 249/333 (74.8%) to 251/333 (75.4%) with no regression.
