bugfix - preserve decorated callable defaults across aliases (#703)

[dannymeijer]

Summary

This PR fixes callable default-argument preservation for decorated functions and methods when the decorated callable surface still matches the source declaration, including direct imports, public facade re-exports, alias re-exports, and alias-expanded Union[...] parameter surfaces. It also moves call-site signature selection toward one canonical merge path so source defaults and imported callable metadata do not diverge across module boundaries.

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: Decorated wrappers keep source default-argument calls when their final callable surface is unchanged. Calls such as imported or re-exported count() no longer lose defaults because the defining signature crossed a module boundary.
• Internals: Function signature resolution now has a shared effective-call-signature merge path. The emitter supplies alias-aware type equivalence so transparent aliases and their expanded surfaces can inherit defaults without a decorator-only special case.
• Risks: This touches function-call and method-call argument shaping, so regressions would most likely appear around default arguments, decorated callables, imported/re-exported functions, or alias-heavy callable signatures.

Testing / verification

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

Manual verification notes:

• cargo test --test cli_integration test_decorated_functions_preserve_default_argument_calls_issue703 --features lsp
• cargo test --test cli_integration test_imported_partial_preset_defaults_survive_decorator_argument_issue698 --features lsp
• cargo test --test cli_integration test_decorator_callable_exposes_source_name_issue694 --features lsp
• cargo test --test cli_integration issue70 --features lsp
• cargo test --test cli_integration issue69 --features lsp
• cargo test --test codegen_snapshot_tests user_defined_decorators
• cargo test --test integration_tests e2e_imported_generic_decorator_factory_preserves_function_signatures
• cargo test --test integration_tests e2e_method_call_decorator_factories_use_checked_receiver_lowering
• git diff --check
• 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/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

Closes #703