feat: add HEAD prefetch cache, and some cleanup

[definfo]

• add mocking test for smart cache following upstream 404 resp
• clean unnecessary FnMut and clippy directives
• simplify error variants, remove unused async allow directives
• add HEAD prefetch cache

Close #72

[Copilot]

Pull request overview

This PR introduces a HEAD-prefetch-based cache to improve “smart-cache” routing decisions (notably returning 404 for genuinely missing objects, per #72) while also cleaning up error variants, clippy directives, E2E fixtures, and Nix build plumbing.

Changes:

• Add a Moka-backed prefetch cache that probes S3 + upstream via HEAD and caches positive/negative existence results.
• Update routing/tests/E2E fixtures to validate 404 behavior and upstream request counts.
• Simplify Rust error variants and refactor Nix flake packaging (removing nix/crane.nix).

Reviewed changes

Copilot reviewed 15 out of 17 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/e2e/simple.sh	Improves E2E assertions (better failure messages) and adds scenarios validating HEAD-prefetch behavior + upstream call counts.
tests/e2e/fake_services.py	Adds request counting endpoint and upstream fixtures to support the new E2E validations.
src/s3_cache.rs	New prefetch cache module (Moka) that performs HEAD probes and caches existence results.
src/repos.rs	Integrates prefetch cache into RouteAction::Cache handling to return 404 when probes confirm missing.
src/common.rs	Extends IntelMission and Config to carry prefetch client/cache and TTL configuration.
src/main.rs	Wires new prefetch client/cache into runtime initialization and adds TTL handling.
src/utils.rs	Updates error variant usage and adjusts tests for the new IntelMission fields.
src/queue.rs	Updates tests to construct IntelMission with prefetch client/cache.
src/storage.rs	Updates simplified error variant usage.
src/browse.rs	Updates simplified error variant usage.
src/artifacts.rs	Updates simplified error variant usage and related matching logic.
nix/crane.nix	Removed (flake now inlines crane args).
flake.nix	Inlines crane args and tweaks build inputs/check behavior; minor comment typo noted.
Cargo.toml	Adds moka dependency for the prefetch cache.
Cargo.lock	Locks new transitive dependencies for moka.
.gitignore	Ignores Python __pycache__/ for E2E helpers.

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