chore - 1480 incan_driver is a crate

[dannymeijer]

Summary

The driver becomes the crate LAYOUT names as the root crate's closest successor. loaves/compiler/incan_driver is src/driver/ as its root — sessions, module collection, typecheck orchestration, Cargo policy, the lock and test-discovery logic, the build pipeline behind the cli feature — with backend/ (the generated project, the shadow comparison, c_abi, and the backend::ir shim over incan_ir/incan_emit), inspect/ (the codegraph and closure analyses), generated_cache, replacement_compatibility and its frozen v0.5 baselines as its modules. cli/commands/shadow_support.rs comes along as shadow_support: it was driver glue that happened to live under the CLI, and the shadow tests calling its prepare_shadow_legacy_materialization were the last backend → cli reach; the CLI keeps a one-line re-export of compare_source_observable for the run command.

The root re-exports every module under its old path — driver, backend, inspect, generated_cache, replacement_compatibility — so the 131 crate::driver::… references in the CLI, the 8 in the LSP, and every incan::driver::…, incan::backend::… and incan::replacement_compatibility::… in tests/ are untouched. Inside the crate the spellings became its own: crate::driver:: → crate::, crate::provider:: → incan_provider::, crate::oven:: → the store or the baker crate, crate::lockfile:: → oven_model::lock::, and the super:: chains that climbed to the old root are absolute paths now.

Facts the split carries, each fixed here:

• The CLI's test runner reads the driver's project-lock collection metrics (reset_project_lock_collection_metrics, project_lock_collection_counts) and its build tests borrow driver::build::test_support; both were cfg(test) and ride the crate's test_support feature now, which the root's dev-dependency turns on. The feature also forwards the provider's, emission's, the frontend's, rust-inspect's and the Oven crates' test-support features, so one flag lights the whole stack.
• driver/lock has a submodule named rust_inspect, so the crate's paths to the rust_inspect crate are spelled ::rust_inspect.
• Seven CARGO_MANIFEST_DIR anchors meant the checkout root (the generated Cargo.toml tests' crates/… paths, the prepare-project test's Cargo.lock, the replacement-compatibility corpus root); they anchor at oven_model::toolchain_layout::development_root(). Two include_str! paths that climbed out of src/ climb three more levels.
• The crate's features mirror the root's, cli included in its defaults: the build pipeline, the report and test-output derives, and the compatibility corpus's checked collection are behind it (12 tests need it), and the root's defaults had it. cargo test -p incan_driver wants CARGO_BIN_EXE_incan pointing at a built CLI, as the root's unit tests always did; the Oven suite exports it for every root.
• The replacement-compatibility registry names repository paths as evidence, and the 3b/3c moves had left them stale (src/frontend/…, src/backend/replacement/…) — something only the generator and committed_projections_match_the_validated_registry see, which the light lane never runs. Re-rooted here with the frozen v0.5 snapshot (its constant, the baseline manifest.json, tests/example_capability_coverage.rs), and the docs inventory regenerated with generate_replacement_compatibility_inventory.
• The CLI layering baseline loses its shadow_support.rs entry; the semantic-string audit classifies fourteen driver-crate files for the first time (backend/{c_abi,project,shadow}, generated_cache, inspect/, replacement_compatibility with its 40 identifier sites, shadow_support) and refreshes one fingerprint whose site now spells oven_rustc::loaf::OVEN_LOAF_ENV.
• The rustdoc prose in the moved files fills the 120-column width (119 once indented, which is where rustfmt re-wraps), as on the earlier PRs in this stack.
• The semantic-string audit's roots and fixture entries follow the files (src/driver, src/backend, src/inspect, src/generated_cache.rs, src/replacement_compatibility*, shadow_support.rs → the crate).

backend/project/{plan,lock_projection,cargo_toml,runner}.rs stay in the driver for now: their moves into oven_rustc and oven_cargo_compat need inversions of their own (the runner's cfg-gated rust-inspect hook, the generated-project stdlib baseline as plan facts) and are the Oven ring's next step, not this crate's.

Type of change

• Bug fix
• New feature
• Refactor / maintenance
• Documentation
• CI / tooling
• RFC (adds/updates docs/RFCs/*)

Area(s)

• Incan Language (syntax/semantics)
• Compiler (frontend/backend/codegen)
• Tooling (CLI/formatter/test runner)
• Editor integration (LSP/VS Code extension)
• Runtime / Core crates (stdlib/core/derive)
• Documentation

Key details

• User-facing behavior: none; codegen snapshots unchanged.
• Internals: one new workspace member of 57k lines, depending on every compiler-ring crate below it, the three Oven crates, incan_codegraph, rust_inspect, clap (optional), serde/toml/toml_edit, sha1/sha2/hex, semver, rustix, tempfile, thiserror, tracing. Cargo.lock gains it. What remains in the root crate is the CLI, the LSP, the binaries and tests/ — the next two steps.
• Risks: the blanket pub is wider than the crate's eventual surface.

Testing / verification

• cargo check -p incan in the six feature configurations; cargo clippy --all-targets default and --all-features with -D warnings; rustdoc gate last
• cargo test -p incan_driver with the CLI built — 412/413, the one miss the macOS TMPDIR canonicalization case (canonical_frames_reenter_the_entry_module_through_a_checked_cycle, identical on dev.5)
• cargo test --test cli_layering_guardrails --test vocab_guardrails --test layering_guard
• cargo test --test codegen_snapshot_tests — 266 unchanged
• make check-oven-ring
• root unit tests with the CLI built for cli::commands::{build,run,oven,tools,lock}, cli::test_runner, lsp — 112/116, the four misses the prewarmed-store build_library_*/inspect_registry_* class that fails identically on the base tree
• one exact Oven root: make test-one TEST_ROOT=tests/cli_surface_tests.rs TEST_EXACT=a_cargo_manifest_beside_a_loaf_manifest_warns_without_stopping_the_build — bake + build

Docs impact

• No docs changes needed (the crate README says what moved and what waits; the ring documentation is step 6)
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

Checklist

• I kept public docs user-focused and moved internals to contributing docs when appropriate
• I avoided duplicating canonical install/run instructions in multiple places
• I added/updated tests where it materially reduces regressions

Part of #1480 (slice 5, #1478). Builds on #1577, #1578, #1579 and #1580: the branch carries their commits, so the diff shows every step until they land and merge-commit syncs narrow it to this one.