Reorganize Rust benchmark file structure following Neo4jVSDoublets pattern

[konard]

Summary

This PR reorganizes the Rust file structure to match the patterns used in Comparisons.Neo4jVSDoublets, making it easier to compare PostgreSQL and Doublets implementations side-by-side.

Fixes #19

Key Changes

1. Benchmark File Structure Reorganization

The benchmarks are now split into two separate modules for clear comparison:

rust/benches/benchmarks/
├── mod.rs              # Main module with documentation and re-exports
├── doublets/           # All Doublets benchmarks
│   ├── mod.rs          # Doublets module documentation
│   ├── create.rs       # Link creation benchmark
│   ├── delete.rs       # Link deletion benchmark
│   ├── update.rs       # Link update benchmark
│   └── each/           # Query benchmarks
│       ├── mod.rs
│       ├── all.rs      # Query all links
│       ├── identity.rs # Query by ID
│       ├── concrete.rs # Query by source+target
│       ├── outgoing.rs # Query by source
│       └── incoming.rs # Query by target
└── psql/               # All PostgreSQL benchmarks
    ├── mod.rs          # PostgreSQL module documentation
    ├── create.rs       # INSERT benchmark
    ├── delete.rs       # DELETE benchmark
    ├── update.rs       # UPDATE benchmark
    └── each/           # SELECT benchmarks
        ├── mod.rs
        ├── all.rs      # SELECT all
        ├── identity.rs # SELECT by ID
        ├── concrete.rs # SELECT by source+target
        ├── outgoing.rs # SELECT by source
        └── incoming.rs # SELECT by target

2. Benched Trait Implementation Split

The Benched trait implementations are now organized in a separate module:

rust/src/benched/
├── mod.rs              # Benched trait definition with documentation
├── doublets_benched.rs # Doublets storage backend implementations
└── psql_benched.rs     # PostgreSQL storage backend implementations

3. Documentation Improvements

• Added comprehensive module-level documentation with operation tables
• Each file includes doc comments explaining the implementation approach
• SQL queries are documented in the PostgreSQL benchmark files
• Time complexity notes are included for both backends

4. Benchmark Organization

• Separate criterion_group! for PostgreSQL and Doublets benchmarks
• Prefixed exports (psql_* and doublets_*) for clear identification
• Parallel structure between psql/ and doublets/ directories

5. Additional Improvements

• Added Rust-specific entries to .gitignore (target/, debug/, *.rs.bk, mutants.out/, etc.) following Neo4jVSDoublets pattern
• Fixed typo in README.md: "diffrence" → "difference"

Comparison with Neo4jVSDoublets

Aspect	Neo4jVSDoublets	PostgreSQLVSDoublets (after)
Backend dirs	neo4j/, doublets/	psql/, doublets/
Operation files	Separate per operation	Separate per operation
Query subdir	each/	each/
Module docs	Detailed tables	Detailed tables
Criterion groups	Separate per backend	Separate per backend
Benched trait	src/benched/ with separate files	src/benched/ with separate files
.gitignore	Rust-specific patterns	Rust-specific patterns ✓

Test Plan

• cargo check passes for library
• cargo check --benches passes for benchmarks
• File structure mirrors Neo4jVSDoublets pattern
• .gitignore includes Rust-specific entries
• README typo fixed
• CI workflow runs successfully

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[konard]

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