bugfix - materialize imported const str arguments (#638)

[dannymeijer]

Summary

This PR fixes imported public const str values at owned Incan str call sites. Public const metadata can arrive as a frozen string shape while the generated Rust item is still &'static str; the backend now treats borrowed/static/frozen string-like IR shapes as owned-string materialization candidates at Incan sinks, and string method receivers use AsRef<str> instead of emitting unstable .as_str() on raw &str constants.

It also bumps the release candidate to 0.3.0-rc8 and records #638 in the v0.3 release notes.

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: from registry import TOKEN where TOKEN is pub const TOKEN: str = "token" now works in str arguments, explicit TOKEN.to_string(), local str assignment, and ordinary string methods.
• Internals: Owned Incan sinks now share a borrowed-string-like helper for StaticStr, StrRef, and FrozenStr, avoiding the prior StaticStr-only materialization gap.
• Risks: FrozenStr receiver lowering now relies on AsRef<str>, which is intentionally broader than .as_str() and covered by both explicit FrozenStr smoke coverage and the imported-const repro.

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_const_str_materializes_at_test_call_sites
• cargo test frozen_str_var_to_string
• target/debug/incan test tests in /private/tmp/incan-638-imported-const-str
• target/debug/incan run -c <FrozenStr smoke>
• git diff --check
• make pre-commit
• make -C /Users/danny/Development/encero/InQL ci INCAN=/Users/danny/Development/encero/tmp/incan-636-api-metadata-decorated-signatures/target/debug/incan

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 #638