feat: implement EXIF metadata extraction

[ankitasahu83964]

Description

This PR introduces EXIF-based metadata extraction support into EnvContext under the vigil-deep feature flag.

It enables extraction of:

• Image camera model (ImgModel)
• GPS coordinates (ImgGps)

The implementation uses kamadak-exif to read EXIF data safely and efficiently, and integrates it into the existing lazy evaluation system using OnceLock.

All computations are strictly feature-gated and do not affect builds where vigil-deep is disabled.

Why this is necessary

This enhancement improves Arbiter’s analytical capability by enabling deeper inspection of image files, allowing downstream workflows to make decisions based on camera metadata and geolocation when available.

Related Issue

Closes #13

Checklist

• I have read the CONTRIBUTING.md guidelines.
• My code passes cargo clippy -- -D warnings and cargo fmt.
• I have included rationale-only comments for complex logic.
• I have added or updated unit tests (if applicable).

AI-Generated Code Compliance

• This PR contains code generated or assisted by an LLM.
• I have manually reviewed, tested, and verified every line of this code. Any issues faced, created or caused by the code are my own fault.
• I can fully explain the low-level mechanics of this implementation and guarantee it does not introduce memory leaks, race conditions, or undefined behavior.

[Sid-352]

Code is decent and wrapped in serde skipped as needed, but there are two issues that need to be fixed before we can merge;

• Performance: rn compute_image_model and compute_image_gps both open the file and parse the EXIF data from scratch. If a decree checks both keys, we parse the file twice. Please consolidate this. Parse the file once, cache a combined struct (e.g., ExifCache { model, gps }) into a single OnceLock, and have the EnvKey matches read from that.
• Dead Code: Please delete all the commented-out //use statements at the top of the file and the commented-out compute_exif_model block at the bottom.

After this ill squash and merge

[Sid-352]

Merged, thanks