Adds two snapshot tests (`nested-headings.md`, `code-and-table.md`) under
crates/kb-parse-md/tests/blocks_snapshots.rs, with matching baseline JSON
next to each fixture. The snapshot view projects `kb_core::Inline` to
flat strings — `Inline` carries `serde(tag = "kind")` which is
incompatible with newtype variants holding a primitive (`Text(String)`),
so direct serialization of `ParsedBlock` would fail today. The view
preserves the contract that matters for P1-3 (heading paths, source
spans, payload kinds, payload text/code/table content) and will keep
working once kb-core fixes the Inline schema in a later task.
Also tightens `level_to_use >= 1 && <= 6` into `(1..=6).contains(&_)` to
satisfy `clippy::manual_range_contains`.
altair823
f604a381df