Run the Bun and vlt vendored preflights as one vendor_preflight with one per-purl refusal map #1839

[agent] Filed by the scheduled architecture audit (audit-cli-tests). Register: register comment.

Kind: refactor. Register ID: T05.

Problem

Every vendored caller runs the Bun preflight and the vlt preflight back to back, then checks each purl against both refusal types with its own glue code:

• agent_download.rs L1255-L1268,`` with glue in VendorRefusals::for_purl (L759)
• get.rs L2326-L2338
• scan/vendor_flow.rs L204-L206 and L356-L358
• vendor.rs L3168-L3174, with glue at L3391-L3408

Both refusal types are {code: &'static str, detail: String}. The two modules (bun_preflight.rs,````` vlt_preflight.rs) duplicate several pieces:

• the npm-only scope check;
• selection_pairs;
• the vendor_state_unreadable mapping;
• the vlt_routes probe, which bun runs to opt out and vlt runs again to opt in.

Bun also exposes three one-line forwarders (bun_vendor_preflight, _with_ledger, _pairs). It has an Option<LedgerLoad> lazy-load mode, and its only caller (get.rs L2326) loads the ledger itself on the next line anyway. The callers also disagree on the root:

• get runs Bun on project_root() but vlt on cwd;
• agent_download runs both on cwd;
• vendor_flow loads the ledger at cwd.

Proposed change

• Add one vendor_preflight(cwd, pairs, ledger) -> HashMap<purl, (code, detail)>. Bun's project-level verdict is expanded per purl after its exemptions.
• Collapse the three Bun forwarders and the lazy-load mode.
• Each caller does one lookup per purl.
• Pick one root, following the #745 rule (the ledger and lock at project_root()).

Size

6 files. About −150 / +60 lines.

Acceptance criteria

• Every Bun/vlt refusal code, and its envelope and exit code, is unchanged.
• The bun_preflight/vlt_preflight unit tests and the e2e_vendor_bun_build/e2e_redirect_vlt_build suites stay green.