[prrao87]

Why

GraphConfig keys its node and relationship mappings by lowercase label so lookups are case-insensitive. The mapping fields were public, so code outside GraphConfigBuilder could insert a key like "Person" and lookups would silently miss it. Deserialization had the same gap: a saved config was loaded without normalizing or validating its keys.

Closes #23. The issue's second half, type checking comparisons and arithmetic in semantic analysis, already landed with the 0.3.0 integration in crates/lance-csr/src/semantic/types.rs. A mismatch in WHERE or RETURN raises a ValueError that names the expression and both operand types, it has tests, and the error table in docs/cypher.md already describes it. This PR doesn't change it.

What changed

• node_mappings and relationship_mappings on GraphConfig are private. Reads go through get_node_mapping, get_relationship_mapping, iter_node_mappings and iter_relationship_mappings, which every caller in the workspace and the Python bindings already used.
• GraphConfig now deserializes through GraphConfigBuilder::build(), so a loaded config is validated like a built one. The serialized format is unchanged.
• The crate README describes the private fields instead of promising a future migration.

Breaking change for Rust users: code that writes to the mapping fields directly no longer compiles and should use the builder instead. A deserialized config that build() would reject now fails to load: one with an unnormalized key, or a relationship whose endpoint label has no node mapping. Because of this, the next release should be a minor version bump. The Python API is unaffected.

Validation

• cargo fmt --all -- --check passes.
• cargo clippy --workspace --all-targets -- -D warnings passes.
• cargo test --workspace passes, including a new round-trip test that also checks an unnormalized key is rejected on load.
• uvx typos@1.26.0 passes on the changed files.
• The Python suite (make test) was not run locally. The bindings compile under clippy and don't touch the fields. CI is pending.

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