Add GitHub Actions CI, pin Rust toolchain, and enable workspace lints; fix clippy issues

[PhantomInTheWire]

Motivation

• Provide a reproducible, CI-enforced Rust quality gate (formatting, linting, tests) for the workspace.
• Pin the local/CI toolchain so rustfmt/clippy behavior is consistent across environments.
• Centralize workspace lint policy and perform minimal code/test adjustments so cargo clippy --workspace --all-targets --all-features -- -D warnings can run under a single policy.

Description

• Add CI workflow at .github/workflows/ci.yml with three jobs: formatting (runs cargo fmt --all --check), clippy (runs cargo clippy --workspace --all-targets --all-features -- -D warnings), and test (runs cargo test --workspace --all-targets --all-features).
• Add rust-toolchain.toml pinned to 1.92.0 and request clippy/rustfmt components so local and CI toolchains match.
• Add workspace-level lints in Cargo.toml ([workspace.lints.clippy]) and opt each crate into the shared lint policy by adding [lints] workspace = true to crate Cargo.tomls.
• Make small code and test hygiene fixes required by Clippy and formatting: collapse a nested if in crates/garuda-segment/src/lib.rs, add #![allow(dead_code)] to test helpers in crates/garuda-engine/tests/common.rs, add an #[expect(...)] justification on a test that must call set_readonly(false), replace a OptimizeOptions::default() call with OptimizeOptions in a test, and apply repository-wide cargo fmt normalization (minor formatting changes in crates/garuda-engine/src/query.rs, crates/garuda-engine/src/validation.rs, garuda/src/main.rs, and test files).

Testing

• Ran cargo fmt --all --check locally and it passed after running cargo fmt --all to normalize formatting.
• Ran cargo clippy --workspace --all-targets --all-features -- -D warnings locally and resolved lints so Clippy now completes under the workspace policy.
• Ran cargo test --workspace --all-targets --all-features locally and observed the existing test failures remain: three failing tests in crates/garuda-engine/tests/test_collection_exception.rs (add_column_rolls_back_state_when_persist_fails, failed_checkpoint_restores_rewritten_segment_files, and failed_persist_does_not_publish_new_manifest_on_reopen), so the CI test job reflects the repository's current behavior rather than masking those failures.

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Codex Task

Summary by CodeRabbit

• New Features

  • Added an is_empty() check for the flat index.

• Chores

  • Added automated CI pipeline for formatting, linting, and tests.
  • Pinned Rust toolchain to 1.92.0 with clippy and rustfmt.
  • Enforced workspace-wide lint settings and upgraded clippy rules to treat certain patterns as errors.

• Refactor

  • Minor internal control-flow and style refinements across modules.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9cf992d3-dc25-4621-85e6-1bd73fc88554

📥 Commits

Reviewing files that changed from the base of the PR and between 5ac47f8 and f6cb3c0.

📒 Files selected for processing (1)

• .github/workflows/ci.yml

✅ Files skipped from review due to trivial changes (1)

• .github/workflows/ci.yml

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📝 Walkthrough

Walkthrough

Adds GitHub Actions CI and pins Rust toolchain to 1.92.0; introduces workspace-level Clippy lints denying dbg_macro and todo; propagates lint inheritance to crate manifests; small test attribute tweaks and minor refactors including a new FlatIndex::is_empty method.

Changes

Cohort / File(s)	Summary
CI & Toolchain \.github/workflows/ci.yml, rust-toolchain.toml	Adds GitHub Actions workflow running cargo fmt, cargo clippy, cargo test on push/PR; pins Rust toolchain to 1.92.0 with clippy and rustfmt components.
Workspace lints Cargo.toml, crates/.../Cargo.toml, garuda/Cargo.toml	Introduces [workspace.lints.clippy] denying dbg_macro and todo; adds [lints] workspace = true to multiple crate manifests so they inherit workspace lint settings.
Crate manifest updates crates/garuda-engine/Cargo.toml, crates/garuda-index-flat/Cargo.toml, crates/garuda-index-hnsw/Cargo.toml, crates/garuda-math/Cargo.toml, crates/garuda-meta/Cargo.toml, crates/garuda-planner/Cargo.toml, crates/garuda-segment/Cargo.toml, crates/garuda-storage/Cargo.toml, crates/garuda-types/Cargo.toml, garuda/Cargo.toml	Each crate now has a [lints] section with workspace = true to opt into workspace lint inheritance.
Tests & annotations crates/garuda-engine/tests/common.rs, crates/garuda-engine/tests/test_collection_exception.rs, crates/garuda-engine/tests/test_collection_optimize.rs	Adds #![allow(dead_code)] to test helper; adds an #[expect(clippy::...)] attribute to one test; adjusts one test call argument from OptimizeOptions::default() to OptimizeOptions.
API addition crates/garuda-index-flat/src/lib.rs	Adds pub fn is_empty(&self) -> bool to FlatIndex.
Refactors / control-flow simplifications crates/garuda-index-hnsw/src/build.rs, crates/garuda-index-hnsw/src/search.rs, crates/garuda-segment/src/index.rs, crates/garuda-segment/src/search.rs	Replaces some explicit return with tail expressions, uses ? for early None return, and consolidates nested conditionals; no behavioral changes.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰
I hopped through code and linted stout,
CI now runs, the checks set out.
No dbg! nor todo left to find,
Small refactors, tidy mind.
A happy rabbit, build passed — hop! 🥕

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 26.32% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately and specifically captures all main objectives: adding GitHub Actions CI, pinning the Rust toolchain, enabling workspace lints, and fixing clippy issues.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/add-ci-for-formatting-and-testing

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