feat: add Cargo/Rust crate patching support behind feature flag

[mikolalysenko]

Summary

• Adds Cargo/Rust ecosystem support behind a cargo feature flag, enabling patching of crates in $CARGO_HOME/registry/src/ and vendor/ directories
• Refactors duplicated ecosystem dispatch logic across 4 CLI commands (apply, rollback, scan, get) into a centralized ecosystem_dispatch module, reducing ~190 lines of boilerplate
• Introduces an Ecosystem enum that replaces string-based ecosystem identification, making future ecosystem additions mechanical

Key changes

Core (socket-patch-core):

• Ecosystem enum in crawlers/types.rs with from_purl(), cli_name(), display_name(), purl_prefix(), all()
• CargoCrawler in crawlers/cargo_crawler.rs — supports registry layout (<name>-<version>/) and vendor layout (<name>/), with line-based Cargo.toml parser and directory-name fallback for version.workspace = true
• Cargo PURL functions in utils/purl.rs (is_cargo_purl, parse_cargo_purl, build_cargo_purl)

CLI (socket-patch-cli):

• ecosystem_dispatch.rs — partition_purls(), find_packages_for_purls(), find_packages_for_rollback(), crawl_all_ecosystems()
• All 4 commands refactored to use dispatch helpers instead of manual per-ecosystem code

Feature flag: --features cargo on both crates; without it, all existing behavior is unchanged.

Test plan

• cargo test --workspace — 233 tests pass (no regressions, no feature flag)
• cargo test --workspace --features cargo — 253 tests pass (20 new cargo-specific tests)
• cargo build --release — clean build without feature
• cargo build --release --features cargo — clean build with feature
• e2e tests verify scan discovers crates in both registry and vendor layouts
• CI should run both cargo test --workspace and cargo test --workspace --features cargo

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