bugfix - materialize imported decorator const str arguments (#638)

[dannymeijer]

Summary

This PR fixes the remaining #638 decorator case where both the decorator factory and const str argument are imported. Decorator factory lowering now gives nested synthetic callees their own neutral span so span-keyed expression metadata for the full factory call cannot overwrite the callee signature needed by argument materialization.

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: Imported decorator factories now accept imported const str arguments in incan test, matching the intended InQL/RFC 014 authoring shape.
• Internals: The full synthetic decorator factory call keeps the source decorator span for typechecker handoff, while its nested synthetic callee uses a neutral span to avoid span-keyed metadata collision.
• Risks: Decorator lowering spans are touched for function and method decorators. Existing decorator codegen coverage plus the new imported factory regression test cover the affected path.

Testing / verification

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

Manual verification notes:

• cargo test e2e_imported_decorator_factory_const_str_argument_materializes --test integration_tests -- --nocapture
• cargo test e2e_imported_const_str_materializes_at_test_call_sites --test integration_tests -- --nocapture
• cargo test user_defined --test codegen_snapshot_tests
• 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): N/A

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

Closes #638