chore - add generated Rust quality gates and async method coverage (#599, #600, #601, #602, #603, #607)

[dannymeijer]

Summary

Adds the generated Rust quality baseline slice for #599-#603 and folds in the #607 async Rust method boundary fix. The PR adds artifact-level generated Rust fixture baselines, stdlib generated Rust snapshots, a strict generated Rust audit runner/gate, clone/allocation hot-path coverage, and package-facing Rust-hosted callability coverage. It also teaches Rust-backed async methods to type as awaitables until awaited and verifies the emitted call includes .await.

Closes #599
Closes #600
Closes #601
Closes #602
Closes #603
Closes #607

Type of change

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

Area(s)

Select the primary areas touched (used for review routing; labels are managed separately):

• 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: generated Rust has new regression gates and documentation for auditing its quality; Rust async method calls now keep Awaitable[T] semantics until explicitly awaited.
• Internals: adds generated artifact, stdlib snapshot, audit, callability, and native-consumer test fixtures; adds make generated-rust-audit-gate; reduces unnecessary generated clone/collection shapes in covered hot paths; extends Rust async metadata handling in typechecking/codegen.
• Risks: the new gates add fixture maintenance cost and may expose generated Rust drift quickly. RFC 097 is only lightly updated to reference the generated Rust quality work; no RFC lifecycle bump is intended in this PR.

Testing / verification

• make test / cargo test
• make examples (if relevant)
• incan fmt --check . (if relevant)
• Manual verification described below

Manual verification notes:

• cargo fmt --all
• cargo test --test generated_rust_callability_artifact_tests
• cargo test --test generated_rust_artifact_tests
• cargo test --test generated_rust_audit_tests
• cargo test --test generated_rust_native_consumer_tests
• cargo test --test stdlib_generated_rust_snapshot_tests
• cargo test --test codegen_snapshot_tests
• cargo test --lib rust_async_method_call --features rust_inspect
• cargo test --lib test_codegen_awaits_async_rust_backed_method_from_metadata --features rust_inspect
• cargo test --lib async_rust --features rust_inspect
• cargo test --lib validate_rust_function_call --features rust_inspect
• cargo test --lib --features rust_inspect -- --test-threads=1
• make generated-rust-audit-gate
• python -m mkdocs build --strict from workspaces/docs-site
• git diff --check origin/main..HEAD
• make pre-commit

Docs impact

• No docs changes needed
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

If docs updated:

• Link(s): workspaces/docs-site/docs/contributing/explanation/generated_rust_quality.md, workspaces/docs-site/docs/contributing/how-to/auditing_generated_rust.md, workspaces/docs-site/docs/contributing/reference/generated_rust_stdlib_coverage.md, workspaces/docs-site/docs/RFCs/097_rust_hosted_incan_caller.md, workspaces/docs-site/docs/release_notes/0_3.md

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