Define public query and batch execution contract

[piraz]

Goal

Define the public query and batch-execution contract required by applications
that use github.com/candango/sqlok.

Context

The core currently exposes SQL AST/compiler pieces plus Mapper and Session
behavior, but application consumers still need a stable public way to execute
filtered lists, joins, aggregates, and batched relationship reads without
copying compiler or executor internals.

Scope

• Define context-aware query execution over caller-owned *sql.DB and
  *sql.Tx boundaries.
• Define row mapping and error behavior for list and empty-result queries.
• Define batch loading/relationship patterns that avoid per-row query fan-out.
• Keep compiler plans, bind buffers, and caches behind the public boundary.
• Add contract tests and documentation before implementation expands.

Non-goals

• Do not mention or depend on a specific downstream application.
• Do not add a global cache or hide transaction ownership.
• Do not import database drivers into the core.

Acceptance criteria

• Public API shape and ownership rules are documented.
• List, empty-result, transaction, mapping-error, and batch-query cases have
  executable contract tests.
• The design preserves driver-agnostic execution and leaves dialect-specific
  behavior to dialects/adapters.