bugfix - unify helper and generic method default planning (#729, #731)

[dannymeijer]

Summary

This PR stacks the #729 vocab helper call-planning fix on top of the merged #727 work and fixes #731 by making instantiated generic receiver methods inherit source default arguments before Rust emission. Vocab desugarer helper calls now use the same public dependency call-planning path as ordinary pub::library calls, including default materialization, dependency-owned provider paths, union wrapping, and owned-string conversion.

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: Source-backed DSL helpers can omit exported defaults the same way ordinary imported helpers can, and calls such as left.join(right) on instantiated generic classes/models now emit the default trailing argument instead of invalid Rust.
• Internals: Public dependency call planning now carries provider-qualified default expressions and nested default-call signatures through .incnlib metadata; module-qualified public helper calls resolve through the same manifest signature path as direct imports; method-call emission specializes Incan-owned receiver signatures through the concrete generic receiver before inheriting defaults.
• Risks: The touched paths sit in shared call lowering/emission and public metadata export. Regression coverage covers direct and vocab helper calls, provider-owned default constants, anonymous-union helper arguments, imported string constants, and generic receiver method defaults.

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 on the stacked bug - vocab helper calls skip normal call planning for defaults and argument coercions #729/bug - Generic class method calls do not fill default arguments #731 branch: 2,546 Rust tests passed, clippy passed, cargo-deny passed, smoke-test-fast passed, examples passed, and benchmark build checks passed.
• Focused final-history checks after rebasing onto merged origin/release/v0.3 passed: cargo test --locked --test integration_tests issue729, cargo test --locked --test codegen_snapshot_tests test_issue731_generic_method_defaults_codegen, and git diff --check origin/release/v0.3...HEAD.
• The minimized bug - Generic class method calls do not fill default arguments #731 repro passed with CARGO_NET_OFFLINE=true target/debug/incan build /private/tmp/incan-731-repro/repro.incn.

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 #729
Closes #731