perf(ci): implement granular steps with persistent workspace cache

[sonesuke]

Overview

This PR optimizes the CI workflow by implementing granular verification steps while ensuring build artifact reuse across containers.

Changes

• Granular Steps: Replaced the monolithic build/verify step with individual steps for cargo build, clippy, test, etc., using the official devcontainers/ci action.
• Persistent Cache: Explicitly mapped CARGO_HOME and CARGO_TARGET_DIR to host-mounted workspace directories. This stabilizes the Rust environment and enables true incremental builds even across separate container invocations.
• Build Order Optimization: Placed cargo build and cargo run -- check sequentially to reuse the same build profile, reducing Docgraph Check time from 20s to 2s.

Results (Run #327)

• Build & Initialize: 120s
• Docgraph Check: 2s (Reused build artifacts)
• Cargo Clippy: 13s (Incremental)
• Tests & Coverage: 36s

This configuration provides maximum UI visibility with optimal performance.