Stage 3 of the faithful libyaml scanner port, continued. Translates fetch_flow_scalar and scan_flow_scalar: single- and double-quoted scalars with line-break folding (a single break folds to a space, blank lines to newlines), the single-quote '' escape, double-quoted backslash escapes (the full \0 \a \b \t \n \v \f \r \e space \" / \ \N \_ \L \P table plus \xXX/\uXXXX/\UXXXXXXXX hex), and the escaped end-of-line continuation that suppresses the break. Escapes are decoded to Unicode scalars directly rather than UTF-8 bytes. Unit-tested for the escaped single quote, the escape table including a hex unicode escape, and multi-line folding. The remaining unported fetchers are block scalars, anchors, tags, and directives. 310 tests, no regression.
Stage 3 of the faithful libyaml scanner port, continued. Translates fetch_flow_collection_start, fetch_flow_collection_end, and fetch_flow_entry (the [ ] { } , indicators), with the flow-level push/pop already ported. Combined with the plain-scalar scanner's flow-aware termination, the port now tokenizes flow sequences and mappings of plain scalars, and the simple-key splice promotes a flow mapping key. Unit-tested for [a, b] and {k: v}. Remaining unported fetchers: quoted/block scalars, anchors, tags, directives. 307 tests, no regression.
Stage 3 of the faithful libyaml scanner port: the token producer. Translates scan_to_next_token, fetch_more_tokens/fetch_next_token dispatch, and the fetchers for stream start/end, document indicators, block entry, key, value (with the simple-key splice and roll-indent), plus scan_plain_scalar with its line-break folding. Reuses the ported indent engine, simple-key mechanism, character classes, and reader. Constructs not yet ported (flow collections, quoted scalars, anchors, tags, directives, block scalars) throw an explicit 'not yet ported' error rather than mis-scan; the port is not yet wired to the public API.
Milestone: the ported scanner tokenizes '- a:' followed by a same-column 'b:' as siblings (one BLOCK-MAPPING-START), matching libyaml, fixing the original block-mapping bug by construction; a more-indented value still nests. Unit-tested for the sibling case, the nested case, a simple pair, and multi-line plain-scalar folding. 305 tests, no regression.
