Harden SST compiler and query derivation #16

Summary

Harden the SQLok SST/compiler boundary after the initial CRUD and compiled-shape cache review.

The current implementation is a solid experimental foundation, but several correctness, security, and performance gaps must be resolved before the cache or public API is treated as production-ready.

Scope

• Derive canonical query shapes from SST structure instead of trusting caller-supplied cache keys.
• Include dialect, compiler version, and relevant model/schema descriptor versions in shape identity.
• Reject stale or structurally incompatible cached statements before execution.
• Define identifier validation and/or dialect-aware quoting for schema, table, and column names.
• Reject SELECT roots without a projection instead of emitting invalid SQL.
• Replace full AST argument traversal on cache hits with a real bind layout when practical.
• Reconcile LIMIT/OFFSET semantics with the documented ANSI portability decision.
• Clean up exported Go documentation and standardize benchmark APIs.

Security requirements

• Runtime values must remain outside SQL templates and cache keys.
• Dynamic identifiers must not be interpolated without validation or quoting.
• A cache hit must never return SQL from a different structural statement.
• Stale artifacts must be invalidated or rejected before an executor is called.

Acceptance criteria

• Equivalent statement structures with different values share a canonical shape.
• Different statement structures cannot collide silently.
• Shape identity accounts for dialect and compiler/model/schema versions where relevant.
• Invalid identifiers fail safely or are rendered through an explicit quoting policy.
• SELECT without expressions returns a construction/compile error.
• Cache-hit benchmarks measure automatic binding rather than a hand-written upper bound only.
• Regression tests cover wrong-key reuse, invalidation, identifier safety, empty SELECTs, and parameter mutation.
• go test ./..., go vet ./..., and relevant race/benchmark gates remain green.

Related work

• Compiled statement cache: Cache compiled SQL statement shapes with parameter validation #15
• SQLok architecture and execution strategy documentation