refactor(p10-3): expose tier2_shared::build_chunk as pub(crate)

Tier 3 chunker (next task) needs to call the same Chunk-construction helper
to keep id / hash / token-count / policy_hash semantics identical with
Tier 2. Visibility-only change; signature and body unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-chunk/src/tier2_shared.rs

@@ -88,7 +88,7 @@ pub(crate) fn push_chunks_with_oversize(
 /// for normal single-chunk emission.  Mirrors the `Some(part_ls)` / `None`
 /// split_key pattern in 1A-2.
 #[allow(clippy::too_many_arguments)]
-fn build_chunk(
+pub(crate) fn build_chunk(
     doc: &CanonicalDocument,
     chunker_version: &ChunkerVersion,
     base_policy_hash: &str,