refactor(core): stop lossy GfError stringification at stage boundaries

[DecisionNerd]

Found during the architecture audit (sub-agent review).

Finding (MEDIUM)

Stages define structured errors (ParseError+kind in ast; BindError{kind,span} ir/binder.rs:111-118; LoweringError{UnknownFunction|UnsupportedExpr|UnboundVar|InvalidType} rel/expr.rs:74-95; AlgorithmError in exec) — but every public entry flattens to GfError strings:

• rel → GfError::Plan(e.to_string()) at every public fn (rel/lowerer.rs:520,550,568,586,606,649); even LoweringError::InvalidType (a validation rejection) surfaces as GF_PLAN
• exec round-trips GfError ↔ DataFusionError: to_df_err (exec:457-458) wraps into DataFusionError::External, then back via e.to_string() (exec:5911,5678) — double-stringification
• Entry-point divergence is tracked separately in the cypher-pure-parser issue

Why it matters

Error taxonomy at the public API is accidental (string content decides the code), and diagnostics lose span/kind information the stages already computed.

Acceptance criteria

• Structured stage errors preserved to the facade boundary (GfError variants carry kind/spans where stages provide them)
• LoweringError::InvalidType no longer surfaces as GF_PLAN (test)
• GfError↔DataFusionError round-trip no longer double-stringifies
• Error-code matrix test enumerating entry point x failure kind

Architecture review decision

Treat this as a parent public-error-contract issue. Define the conversion matrix before changing individual call sites.

Additional acceptance criteria:

• One matrix covers stage error kind/span -> GfError -> Python exception -> Node error code
• Existing stable public codes are preserved unless a versioned release decision explicitly changes them
• DataFusion wrapping preserves a downcastable structured source when it originated as GfError
• refactor(cypher): make graphforge-cypher a pure parser crate #1007 consumes this mapping rather than defining a second binder-error policy

The release impact is intentionally left undecided until public-code compatibility is proven.