docs: fix stale --features gpu references

[Karanjot786]

ARCHITECTURE.md tells you to build the GPU backend with --features gpu on the umbrella crate. No such feature exists. crates/compass/Cargo.toml declares one opt-in feature, object-storage, and carries no dependency on compass-vector-gpu.

Line 66 of the same file already states this, added by the v0.4 docs truth sweep (1f53904). Two earlier mentions stayed behind. The crate docs in compass-vector-gpu/src/lib.rs show a second broken snippet:

compass = { version = "0.2", features = ["gpu"] }

Wrong feature, and the workspace sits at 0.4.0.

Changes

• ARCHITECTURE.md:19, point readers at a direct dependency on compass-vector-gpu. Adds a note on the gpu feature in compass-index-api, which toggles the gpu_available() helper and pulls in no backend.
• ARCHITECTURE.md:101, GPU acceleration today covers the embedding step. The CAGRA build path lives in the unwired crate.
• compass-vector-gpu/src/lib.rs:18, replace the snippet with a git dependency on the crate.

Docs only. No behavior change.

Checks

• cargo fmt --all --check clean
• cargo clippy --workspace --exclude compass-vector-gpu --all-targets -- -D warnings clean
• cargo test --workspace --exclude compass-vector-gpu: 109 passed, 0 failed

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[greptile-apps]

Greptile Summary

Corrects stale GPU setup documentation by directing embedded users to the standalone GPU crate and clarifying that GPU index construction is not integrated into the main engine.

• Removes references to the nonexistent umbrella gpu feature.
• Updates the GPU crate’s dependency example to use its repository package directly.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains within the eligible follow-up review scope.

Important Files Changed

Filename	Overview
ARCHITECTURE.md	Replaces stale GPU feature instructions and clarifies the current separation between the engine and standalone GPU backend.
crates/compass-vector-gpu/src/lib.rs	Updates the crate-level Cargo example to depend directly on compass-vector-gpu.

_{Reviews (2): Last reviewed commit: "docs: fix stale --features gpu reference..." | Re-trigger Greptile}

[Karanjot786]

@EdgarBabajanyan mind taking a look? Small docs fix. CI needs your approval to run.

[EdgarBabajanyan]

running it, thanks @Karanjot786

[Karanjot786]

@EdgarBabajanyan

Pushed the sign-off, dco should pass now.

The audit failure is unrelated to this PR. It touches two doc lines and no lockfile.
Three new advisories landed against the existing tree: h2 0.4.15 (RUSTSEC-2026-0258, fixed in 0.4.16), lru 0.12.5 (RUSTSEC-2026-0253), and chacha20 0.10.1 is yanked.
Happy to open a separate PR for those.