[Query] Clean up and decouple internal Kgram engine modules in corese-next #617

Summary

Clean up, modernize, and clarify internal module boundaries inside core.next.query.impl.engine, removing dead code, harmonizing exception hierarchies, and enforcing strict typing.

Objectives and Technical Scope

1. Package Reorganization and Boundary Clarification

• Review internal packages in core.next.query.impl.engine (eval, event, filter, model, path, pattern, solution, sorter, spi, storage).
• Clarify boundaries between planning, optimization, and runtime execution.

2. Dead Code and Legacy Pruning

• Identify and remove unused classes, deprecated methods, and redundant abstractions left over from the initial engine import.
• Modernize variable names and internal terminology to match contemporary standards.

3. Unified Error Handling and Typing

• Standardize query evaluation exceptions into a coherent hierarchy rooted in core.next.query.api.exception.
• Ensure strict, pervasive usage of DatatypeValue across all engine internal state.

Acceptance Criteria

[ ] Dead code and unused classes in next.query.impl.engine are eliminated.
[ ] Exception hierarchy is clean, coherent, and documented.
[ ] Architecture boundary tests (NextModuleBoundaryTest) pass with zero violations.
[ ] 100% test pass rate across existing unit and integration tests.