Arena implementation (#665)

## Description
<!-- Provide a brief description of the changes in this PR -->

## Related Issues
<!-- Link to any related issues using #issue_number -->

Closes #670 #666 #667 #672  #668 #661 #655 #654 #652 #436 

## Checklist when merging to main
<!-- Mark items with "x" when completed -->

- [ ] No compiler warnings (if applicable)
- [ ] Code is formatted with `rustfmt`
- [ ] No useless or dead code (if applicable)
- [ ] Code is easy to understand
- [ ] Doc comments are used for all functions, enums, structs, and
fields (where appropriate)
- [ ] All tests pass
- [ ] Performance has not regressed (assuming change was not to fix a
bug)
- [ ] Version number has been updated in `helix-cli/Cargo.toml` and
`helixdb/Cargo.toml`

## Additional Notes
<!-- Add any additional information that would be helpful for reviewers
-->

<!-- greptile_comment -->

<h2>Greptile Overview</h2>

Updated On: 2025-11-07 00:19:04 UTC

<h3>Greptile Summary</h3>


This PR implements arena-based memory allocation for graph traversals
and refactors the worker pool's channel selection mechanism.

**Key Changes:**
- **Arena Implementation**: Introduced `'arena` lifetime parameter
throughout traversal operations (`in_e.rs`), replacing owned data with
arena-allocated references for improved memory efficiency
- **Worker Pool Refactor**: Replaced `flume::Selector` with a
parity-based `try_recv()`/`recv()` pattern to handle two channels
(`cont_rx` and `rx`) across multiple worker threads
- **Badge Addition**: Added Manta Graph badge to README

**Issues Found:**
- **Worker Pool Channel Handling**: The new parity-based approach
requires an even number of workers (≥2) and uses non-blocking
`try_recv()` followed by blocking `recv()` on alternating channels.
While this avoids a true busy-wait (since one `recv()` always blocks),
the asymmetry means channels are polled at different frequencies,
potentially causing channel starvation or unfair scheduling compared to
the previous `Selector::wait()` approach.

The arena implementation appears solid and follows Rust lifetime best
practices. The worker pool change seems to be addressing a specific
issue with core affinity (per commit `7437cf0f`), but the trade-off in
channel fairness should be monitored.

<details><summary><h3>Important Files Changed</h3></summary>



File Analysis



| Filename | Score | Overview |
|----------|-------|----------|
| README.md | 5/5 | Added Manta Graph badge to README - cosmetic
documentation change with no functional impact |
| helix-db/src/helix_engine/traversal_core/ops/in_/in_e.rs | 5/5 |
Refactored to use arena-based lifetimes ('arena) instead of owned data,
replacing separate InEdgesIterator struct with inline closures for
better memory management |
| helix-db/src/helix_gateway/worker_pool/mod.rs | 3/5 | Replaced flume
Selector with parity-based try_recv/recv pattern requiring even worker
count, but implementation has potential busy-wait issues that could
cause high CPU usage |

</details>


</details>


<details><summary><h3>Sequence Diagram</h3></summary>

```mermaid
sequenceDiagram
    participant Client
    participant WorkerPool
    participant Worker1 as Worker (parity=true)
    participant Worker2 as Worker (parity=false)
    participant Router
    participant Storage

    Client->>WorkerPool: process(request)
    WorkerPool->>WorkerPool: Send request to req_rx channel
    
    par Worker1 Loop (parity=true)
        loop Every iteration
            Worker1->>Worker1: try_recv(cont_rx) - non-blocking
            alt Continuation available
                Worker1->>Worker1: Execute continuation function
            else Empty
                Worker1->>Worker1: Skip (no busy wait here)
            end
            Worker1->>Worker1: recv(rx) - BLOCKS until request
            alt Request received
                Worker1->>Router: Route request to handler
                Router->>Storage: Execute graph operation
                Storage-->>Router: Return result
                Router-->>Worker1: Response
                Worker1->>WorkerPool: Send response via ret_chan
            end
        end
    end
    
    par Worker2 Loop (parity=false)
        loop Every iteration
            Worker2->>Worker2: try_recv(rx) - non-blocking
            alt Request available
                Worker2->>Router: Route request to handler
                Router->>Storage: Execute graph operation
                Storage-->>Router: Return result
                Router-->>Worker2: Response
                Worker2->>WorkerPool: Send response via ret_chan
            else Empty
                Worker2->>Worker2: Skip (no busy wait here)
            end
            Worker2->>Worker2: recv(cont_rx) - BLOCKS until continuation
            alt Continuation received
                Worker2->>Worker2: Execute continuation function
            end
        end
    end

    WorkerPool-->>Client: Response
```
</details>


<!-- greptile_other_comments_section -->

<!-- /greptile_comment -->

Author: xav-db

.dockerignore

@@ -32,4 +32,4 @@ jobs:
     - name: Run tests
       run: | 
         cd helix-db
-        cargo test --profile dev helix_engine
+        cargo test --release --lib -- --skip concurrency_tests

.github/workflows/db_tests.yml

@@ -6,3 +6,6 @@ test/
 .claude/
 hack/
 .DS_Store
+/tmp
+/data
+claude.md