Trait coherence: disallow inherent impls on external types

[tritao]

This improves the trait coherence checks for impl self case, and polishes a bit of the wording on the existing diagnostics.

We now enforce package-level check in impl-self type checking, which means we now reject inherent impls for external nominal types (struct/enum).

There is still is a specialized whitelist for StorageKey so current code that uses this pattern keeps working.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

---

Note

Enforces package-level coherence by rejecting inherent impl blocks for external nominal types (with a whitelist for std::storage::StorageKey<_>), adds a dedicated diagnostic, and updates tests and orphan rule wording from module to package.

• Semantic analysis:
  • Disallow inherent impl for external nominal types in type_check_impl_self (sway-core/src/semantic_analysis/ast_node/declaration/impl_trait.rs).
  • Temporary whitelist for std::storage::StorageKey<_>.
  • Emit new error CompileError::InherentImplForExternalType with spans.
• Diagnostics (sway-error/src/error.rs):
  • Add InherentImplForExternalType variant and full diagnostic with hints/help.
  • Change orphan rule wording from "module" to "package" in messages and help.
• Tests:
  • Update failing snapshots and checks to expect package-level wording and the new inherent-impl error.
  • Refactor should_pass/language/generic_transpose to use a new trait OptionTranspose instead of an inherent impl on Option<_>.
  • Adjust impl_self_overlap and orphan rule snapshots accordingly.

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[codspeed-hq]

CodSpeed Performance Report

Merging #7385 will not alter performance

_{Comparing tritao:improve-trait-coherence-checks (770e69e) with master (4a44a36)}

Summary

✅ 25 untouched

[tritao]

Rebased due to conflicts.

[cursor]

PR Summary

Enforces package-level orphan rule for inherent impls, adds a new warning/diagnostics (with a std::StorageKey exception), and updates/extends tests accordingly.

• Semantic analysis (impl self):
  • Disallow inherent impls for external struct/enum types; emit Warning::InherentImplForExternalType and abort type checking; whitelist std::storage::storage_key::StorageKey<_>.
• Diagnostics:
  • Orphan-rule wording updated from "module" to "package" (IncoherentImplDueToOrphanRule).
  • Add Warning::InherentImplForExternalType with formatted diagnostic, hints, and future-hard-error help.
• Tests:
  • Add failing case trait_coherence/impl_alias_external and adjust impl_self_overlap to compile with expected warnings.
  • Update orphan-rule snapshot messages to "package".
  • Add passing case trait_coherence/impl_alias_same_package.
  • Refactor language/generic_transpose to use a trait (OptionTranspose) instead of an inherent impl on Option<MyResult<..>>.

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