docs: Add module documentation to query_planner submodules

[genezhang]

Added module-level documentation (//! doc comments) to all files in:

• logical_plan/: 12 files documented (mod.rs, errors.rs, plan_builder.rs, etc.)
• optimizer/: 7 files documented (mod.rs, errors.rs, filter_push_down.rs, etc.)
• plan_ctx/: 2 files documented (mod.rs, errors.rs)
• logical_expr/: 1 file documented (errors.rs)

Also fixed:

• Removed unused imports: Direction in join_builder.rs, plan_builder.rs
• Removed unused re-exports in match_clause/errors.rs

All 832 tests passing. Clippy warnings reduced from 503 to 501.

[Copilot]

Pull request overview

This PR adds comprehensive module-level documentation to the query planner submodules, improving code discoverability and maintainability. The documentation covers 22 files across logical_plan, optimizer, plan_ctx, and logical_expr modules with detailed explanations of purpose, architecture, and usage.

Changes:

• Added module-level documentation (//! doc comments) to 22 files across query_planner submodules
• Removed unused Direction imports from render_plan (plan_builder.rs, join_builder.rs)
• Converted match_clause/errors.rs from re-exports to a placeholder module with documentation

Reviewed changes

Copilot reviewed 25 out of 25 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/render_plan/plan_builder.rs	Removed unused Direction import
src/render_plan/join_builder.rs	Removed unused Direction import (locally imported where needed)
src/query_planner/plan_ctx/mod.rs	Added comprehensive documentation covering PlanCtx, TableCtx, VariableRegistry, and scope chain
src/query_planner/plan_ctx/errors.rs	Added documentation for context operation errors
src/query_planner/optimizer/projection_push_down.rs	Added documentation explaining projection pushdown strategy with example
src/query_planner/optimizer/optimizer_pass.rs	Added documentation for OptimizerPass trait and result types
src/query_planner/optimizer/mod.rs	Added documentation with optimization passes table and execution order
src/query_planner/optimizer/filter_push_down.rs	Added documentation explaining filter pushdown strategy with example
src/query_planner/optimizer/filter_into_graph_rel.rs	Added documentation for filter-to-GraphRel embedding optimization
src/query_planner/optimizer/errors.rs	Added documentation for optimizer error types
src/query_planner/optimizer/cleanup_viewscan_filters.rs	Added detailed documentation with execution order and example
src/query_planner/logical_plan/with_clause.rs	Added documentation covering WITH semantics, SQL translation, and differences from RETURN
src/query_planner/logical_plan/where_clause.rs	Added documentation for WHERE clause processing and union handling
src/query_planner/logical_plan/unwind_clause.rs	Added documentation for UNWIND clause with SQL translation and use cases
src/query_planner/logical_plan/skip_n_limit_clause.rs	Added documentation for SKIP/LIMIT pagination clauses
src/query_planner/logical_plan/return_clause.rs	Added documentation covering RETURN features and aggregation handling
src/query_planner/logical_plan/projection_view.rs	Added documentation for Projection-ViewScan integration utilities
src/query_planner/logical_plan/plan_builder.rs	Added comprehensive documentation with clause processing order and error handling
src/query_planner/logical_plan/order_by_clause.rs	Added documentation for ORDER BY clause processing
src/query_planner/logical_plan/optional_match_clause.rs	Added documentation explaining OPTIONAL MATCH LEFT JOIN semantics and implementation
src/query_planner/logical_plan/mod.rs	Added extensive documentation covering architecture, key components, plan building, and examples
src/query_planner/logical_plan/match_clause/errors.rs	Converted from re-exports to placeholder with explanatory documentation
src/query_planner/logical_plan/filter_view.rs	Added documentation for Filter-ViewScan integration utilities
src/query_planner/logical_plan/errors.rs	Added documentation for logical plan construction errors
src/query_planner/logical_expr/errors.rs	Added documentation for logical expression conversion errors

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