bugfix - preserve by-value generic Rust call arguments (#609)

[dannymeijer]

Summary

This PR fixes Rust interop call emission for metadata-backed associated functions whose inspected signature takes a generic parameter by value, such as decode<T: Buf>(buf: T). The typechecker now preserves plain Rust type variables as exact by-value call-site parameters and the backend honors that explicit external signature, so owned cursor arguments are passed directly instead of being fallback-borrowed.

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: Rust interop calls to prost-style decode methods, for example FileDescriptorSet.decode(cursor), now pass owned cursor values directly when the Rust signature accepts T: Buf by value.
• Internals: Rust boundary matching now treats direct Rust type variables as exact by-value parameter matches, records exact call-site parameter shape only when inspected params contain TypeVar, and method emission keeps explicit external generic params on the external call-arg path.
• Risks: The touched paths affect Rust interop argument shaping. The main risk is a regression in borrow/coercion decisions for external Rust calls; focused regressions and the full local gate cover the by-value generic path and adjacent Rust interop behavior.

Testing / verification

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

Manual verification notes:

• make pre-commit passed after the final commit, including formatting, rustdoc, 2435 nextest tests, LSP focused tests, clippy, cargo-deny, and smoke-test-fast.
• Focused regression coverage added for frontend Rust boundary metadata, backend method emission, and an end-to-end CLI project using a local decode<T: DecodeBuf>(T) helper crate.

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 #609