feat(p10-2): manifest-file-v1 chunker (whole-file 1 chunk, symbol <manifest>)

Emits 1 Chunk per manifest file (Cargo.toml / pyproject.toml / package.json /
tsconfig.json / pom.xml / build.gradle / go.mod). Symbol unified to
"<manifest>"; manifest type distinguished by code_lang (toml / json / xml /
groovy / go-mod) read from Block::Code.lang. Oversize >200 lines splits via
tier2_shared::push_chunks_with_oversize.

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

crates/kebab-chunk/tests/fixtures/sample_cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "demo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+serde = "1"