fix(cargo): fully migrate hosted<->vendored modes (C1-C7) - #196

What

Makes a cargo mode migration (vendored ↔ hosted) a real takeover: the flow reverts the displaced mode's wiring per purl BEFORE applying the new mode, so the project ends FULLY in the new mode — or the purl is refused with an actionable error. Fixes audit findings C1–C7.

• Hosted scan over a vendored crate (C1): run_redirect now runs the exact per-purl vendor --revert machinery first (restore the Cargo.lock originals from the vendored ledger, drop the [patch.crates-io] entry, remove the committed tree and the ledger entry), persisting the ledger per purl, and only then redirects. A purl whose vendored state cannot be cleanly reverted (revert failure, ledger unreadable/unwritable) is refused — skipped with redirect_vendored_revert_failed — never half-migrated. Dry runs preview the takeover with redirect_would_revert_vendored.
• Vendored flows over a hosted redirect (C2, C7): both the standalone vendor command and scan --mode vendored now revert the purl's hosted edits from the redirect ledger's recorded FileEdits first — Cargo.toml loses its registry = "socket-patch-…" pin, Cargo.lock gets the original crates.io source/checksum back, the now-unused [registries.socket-patch-…] block is dropped — then drop the ledger record and vendor. New core module patch/redirect/takeover.rs (revert_cargo_redirect_purl) unwinds re-redirect chains newest-first and fails closed on drifted files (neither the recorded redirected fragment nor the original matches ⇒ Err ⇒ the purl is refused with redirect_revert_failed). The plain vendor command also emits the same vendor_supersedes_redirect envelope advisory the scan flow does (C7).
• Missing/corrupt ledger backstop: vendor_cargo_crate itself refuses (hosted_redirect_live) when live hosted wiring is present with no redirect-ledger record to revert it — with manual remediation steps — instead of baking the hosted registry values into the vendor ledger as "originals".
• Lock-originals preservation (C3): the pre-revert ordering means the vendor detach records the PRISTINE crates.io fragment; additionally persist_vendor_entry carries the cargo lock originals forward on re-vendor (entry.lock was previously dropped whenever the re-detach found nothing to detach), so an A→B→A round trip can no longer overwrite the ledger's only copy of the unrecoverable crates.io source/checksum, and vendor --revert restores the true pre-vendor lock.
• Classifier cargo blindness (C4/C4b): classify_overlap_takeover gains a cargo-specific probe (classify_cargo_overlap) that reads the Cargo.lock entry's actual shape — source matching a config-declared [registries.socket-patch-*] sparse index (or patch.socket.dev) proves hosted; a detached entry with [patch.crates-io] pointing into the entry's committed .socket/vendor/cargo/<uuid>/ copy proves vendored; anything else stays silent. The previous generic probe could never prove hosted for cargo (lock_inventory records resolved: None), so it inverted the direction and told users to delete the live redirect ledger.
• GC/prune reclamation (C5): dispatch_in_use_one gains the cargo probe (vendored_entry_in_use): a detached lock entry routed through the entry's [patch] copy is in use; a registry-sourced or missing entry is reclaimable, so a vendored entry displaced by a hosted takeover no longer survives every scan --prune forever.
• Remediation text (C6): both takeover warnings now recommend only the safe sequence (vendor --revert / re-run the vendored flow) and explicitly warn against hand-deleting .socket/vendor/ (the [patch.crates-io] entry may still reference it — deleting it hard-fails every cargo command) or redirect-state.json (it may hold the only revert data and VEX records for other still-redirected packages).

Why

Before this change, every cargo mode migration exited 0 while leaving the project broken — the exact fail-open bug class the audit flagged:

• vendored→hosted left both wirings in place; cargo refuses to update the lock over the now-unused [patch] entry, so every CI cargo build --locked fails (C1) — with warnings: [] because the classifier could not see hosted-liveness for cargo (C4);
• hosted→vendored left the Cargo.toml registry pin that [patch.crates-io] cannot patch over — unbuildable online and offline (C2), and completely silent via the primary vendor entry point (C7);
• A→B→A silently destroyed the crates.io lock originals, the one piece of state documented as "not recoverable offline; the ledger entry is their only home", so revert restored a dead grant-tokenized sparse-index URL (C3);
• the inverted warning actively advised deleting the live hosted ledger — destroying the VEX record store and the only hosted revert path (C4b/C6);
• and nothing (GC, prune, repair) could ever reclaim the stale vendored residue (C5).

Testing

New real-binary + real-cargo migration suite crates/socket-patch-cli/tests/mode_migration_cargo.rs (adapted from the audit probes in mode_migration_cargo_audit.rs), red on the pre-fix code and green here. Each scenario proves the terminal state with a fresh-checkout cargo build --locked:

• vendored_then_hosted_takeover_leaves_pure_hosted — vendored state fully gone (no [patch], no tree, no ledger entry), redirect live, fresh checkout builds --locked (C1/C4);
• hosted_then_vendored_takeover_leaves_pure_vendored — registry pin and [registries] block gone, redirect record dropped, fresh checkout builds --locked --offline (C2/C7);
• double_takeover_a_b_a_preserves_lock_originals — ledger keeps the crates.io fragment through A→B→A, and vendor --revert restores the pristine lock byte-identically (C3);
• vendor_over_hosted_without_ledger_is_refused — live hosted wiring with a deleted ledger: vendor refuses (hosted_redirect_live), project untouched (fail-closed).

Plus a GC unit test (vendor_gc_reclaims_cargo_entry_displaced_by_hosted_takeover, C5) and classifier/remediation coverage in the scan tests.

Gates run locally, all green:

• cargo test -p socket-patch-cli --all-features --test mode_migration_cargo (4/4, no skips)
• cargo test -p socket-patch-cli --all-features --test in_process_redirect --test rollback_invariants --test repair_invariants --test e2e_vendor_cargo_build
• cargo clippy --workspace --all-features -- -D warnings
• cargo test --workspace --all-features