[agent] Filed by the scheduled architecture audit routine (ecosystems and formats). Register: E15.

Kind: refactor. Source: review §1 recommendation 1 and Part 5.4 ("Cargo"); register E15.

Problem

Five places read a Cargo.toml [package] table, with three different parsers. Two of them are hand-rolled line scanners, although toml_edit is already a dependency of socket-patch-core.

They have drifted. I ran one set of manifests through the crawler, VEX product detection and cargo_tag on 045d7ec, in a throwaway integration test that I ran twice and did not commit. toml_edit parses all of these inputs:

So a crate whose manifest cargo_tag reads can be invisible to the crawler, and the crawler accepts a manifest that cargo rejects. In a cargo vendor directory the crawler's fallback (the directory name) carries no version, so such a crate cannot be discovered at all. The VEX scanner and the crawler also disagree with each other on BOMs.

Normalized crates.io manifests always use a plain [package] table, so real registry crates hit none of these rows today. The point of this issue is the duplicate parsers and the rules that have already drifted, not a live wrong answer.

Symptoms

• Vendored cargo warns cargo_multi_version_old_cargo ("declares no rust-version") when the root package inherits rust-version from [workspace.package] #651: declared_cargo_minor reads [package].rust-version with an ad hoc toml_edit lookup and misses rust-version.workspace = true. A shared reader that models workspace inheritance ({ workspace = true } → [workspace.package]) fixes this once for version and rust-version alike.

Impact: low risk, small size. Deleting the hand-rolled scanner removes a class of "valid TOML the scanner misreads" bugs (the crawler has already needed fixes for header comments and single quotes, according to its own comments).

Proposed change

Add one pure reader in formats/cargo/manifest.rs (or formats/cargo/mod.rs), built on toml_edit:

pub(crate) struct PackageFields { name: Option<String>, version: Inherit<String>, rust_version: Inherit<String>, workspace: Option<String> }
pub(crate) enum Inherit<T> { Literal(T), Workspace, Absent }
pub(crate) fn package_fields(doc: &DocumentMut) -> Option<PackageFields>   // [package], else legacy [project]

Then:

• Delete parse_cargo_toml_name_version, parse_table_header and extract_string_value from cargo_crawler.rs (about 100 lines plus their unit tests). The crawler parses with toml_edit (BOM handled by the parser) and keeps its directory-name fallback for Inherit::Workspace. tests/crawler_cargo_e2e.rs and cargo_crawler/oracle.rs switch to the shared reader.
• vex/product.rs parse_cargo_toml uses package_fields (scan_toml_section stays for pyproject.toml, which is E38's).
• cargo_tag::version_literal takes its item from the shared lookup and keeps only the span and quote logic.
• path_crate_version uses the shared reader. Its [project] blind spot goes away.

plan_cargo_toml's regex dependency scanner in patch/redirect/mod.rs is the other half of E15. It is out of scope here and will be filed separately.

Size and scope

• formats/cargo/ (new reader, about 60 lines), crawlers/cargo_crawler.rs (−100), vex/product.rs, vendor/cargo_tag.rs, vendor/cargo.rs.
• Estimated diff: about 150 production lines added, 130 removed, plus tests.
• Out of scope: the hosted plan_cargo_toml scanner, pyproject parsing, Cargo.lock (already shared through formats::cargo), and the Vendored cargo warns cargo_multi_version_old_cargo ("declares no rust-version") when the root package inherits rust-version from [workspace.package] #651 warning text. Vendored cargo warns cargo_multi_version_old_cargo ("declares no rust-version") when the root package inherits rust-version from [workspace.package] #651 can land here if the reader models rust-version inheritance; otherwise it stays separate.

Acceptance criteria

• One [package] reader in formats/cargo. No hand-rolled TOML scanner reads Cargo.toml in crawlers/ or vex/product.rs.
• A table test over the rows above (plain, BOM, [project], dotted keys, inline package = { … }, version.workspace = true, invalid TOML): crawler, VEX product and cargo_tag agree on every row.
• version.workspace = true still falls back to the directory name in the registry layout (existing crawler tests).
• Green: cargo test -p socket-patch-core (crawler_cargo_e2e, the cargo_crawler oracle, vex::product, vendor::cargo_tag, vendor::cargo) and cargo clippy --workspace --all-targets.

Dependencies

• Rebase over Read NuGet and Cargo crawler project files through the FIFO-safe reader (#592) #602 (FIFO-safe reads in cargo_crawler.rs). It touches the same read call sites but not the parser.
• It blocks nothing. It is a step toward review recommendation 1 (one codec per format).