Phase 3: Datalog Core Implementation - #4

Add Entity-Attribute-Value (EAV) data model types alongside existing
property graph types as first step of Datalog implementation.

## New Types

### Core EAV Types:
- `EntityId` - UUID-based entity identifiers
- `TxId` - u64 transaction IDs for versioning
- `Attribute` - String-based attribute names (`:person/name` style)
- `Value` - Extended enum supporting:
  - String, Integer, Float, Boolean (like PropertyValue)
  - Ref(EntityId) - NEW: References to other entities
  - Keyword(String) - NEW: Keywords like `:status/active`
  - Null
- `Fact` - Core EAV triple: (Entity, Attribute, Value, TxId, Asserted)

### Fact Structure:
- Immutable facts with transaction tracking
- `asserted` flag for retractions (instead of deletions)
- Support for assertion and retraction operations
- Maintains full history for temporal queries

## Implementation Details

- Added comprehensive accessor methods for Value enum
- Factory methods for Fact creation (new, retract, with_asserted)
- Full serde support for serialization
- PartialEq implementations for testing

## Tests Added (6 new tests):
- `test_value_creation_and_accessors` - All Value variants
- `test_fact_creation` - Basic fact assertions
- `test_fact_retraction` - Fact retractions
- `test_fact_with_ref_value` - Entity references (graph edges)
- `test_fact_equality` - Fact comparison logic
- `test_value_types` - Serialization/deserialization

## Migration Strategy

Property graph types (Node, Edge, PropertyValue) preserved for
backward compatibility during Phase 3 transition. EAV types added
alongside, exported from lib.rs with clear comments.

All 9 tests passing (3 existing + 6 new).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Change TxId from simple counter to timestamp (milliseconds since UNIX
epoch) for natural chronological ordering and better Phase 4 preparation.

## Changes

- TxId now represents milliseconds since UNIX epoch
- Added helper functions:
  - `tx_id_now()` - Get current timestamp as TxId
  - `tx_id_from_system_time()` - Convert SystemTime to TxId
  - `tx_id_to_system_time()` - Convert TxId back to SystemTime
- Exported helpers from lib.rs for public use

## Benefits

1. **Natural ordering** - TxIds are chronologically ordered by default
2. **Human-readable** - Can convert to datetime for debugging
3. **Phase 4 ready** - Consistent with bi-temporal valid_time (also timestamps)
4. **No collision** - Millisecond precision sufficient for Phase 3 single-threaded usage

## Tests Added

- `test_tx_id_timestamp` - Verifies chronological ordering and round-trip conversion
- `test_tx_id_ordering` - Confirms sequential TxIds are strictly increasing

All 11 tests passing.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

This commit implements the foundational Datalog query engine for Minigraf,
marking the completion of core Phase 3 functionality as outlined in the roadmap.

## EAV Data Model
- Add `Fact` struct: (Entity, Attribute, Value, TxId, Asserted)
- Extend `Value` enum with `Ref` and `Keyword` variants for entity references
- Implement timestamp-based transaction IDs (milliseconds since UNIX epoch)
- Add helper functions: `tx_id_now()`, `tx_id_from_system_time()`, `tx_id_to_system_time()`

## Fact Storage Layer
- Implement `FactStorage` with append-only log architecture
- Support assertions and retractions (immutable history)
- Add `transact()` and `retract()` with automatic timestamping
- Implement query methods: `get_current_value()`, `get_facts_by_entity()`, etc.
- Add 8 comprehensive tests for fact storage

## EDN/Datalog Parser
- Full EDN tokenizer supporting vectors, lists, keywords, symbols, UUIDs
- Parse Datalog commands: `(transact [...])`, `(query [...])`, `(retract [...])`
- Support for all value types: strings, integers, floats, booleans, keywords, UUIDs
- Add 11 tests for parser functionality

## Pattern Matching Engine
- Implement `PatternMatcher` with variable unification
- Support multi-pattern queries with variable binding across patterns
- Join algorithm for matching multiple patterns simultaneously
- Convert between EDN values and storage values
- Deterministic keyword-to-UUID conversion for entity references
- Add 6 tests for pattern matching

## Query Executor
- Implement `DatalogExecutor` for command execution
- Execute transact/retract operations with proper error handling
- Query execution with pattern matching and variable projection
- Return structured query results
- Add 6 tests for executor functionality

## REPL Integration
- Update REPL to support Datalog syntax (parenthesis-based commands)
- Maintain backward compatibility with GQL syntax
- Pretty-print Datalog query results with column headers
- Add visual feedback for transactions/retractions

## Public API
- Export all Datalog types and functions from `src/lib.rs`
- Export fact storage and EAV types
- Provide complete API for library users

## Test Coverage
- 81 tests passing (up from 51 before Phase 3)
- Comprehensive coverage across all new modules

## Example Usage
```datalog
(transact [[:alice :person/name "Alice"]
           [:alice :person/age 30]])

(query [:find ?name ?age
        :where [?e :person/name ?name]
               [?e :person/age ?age]])
```

This implementation provides the foundation for Phase 4 (bi-temporal support)
and Phase 5 (ACID + WAL).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>