Make Dart native builds reproducible

[ethicnology]

• I have disclosed my use of AI in the body of this PR.
• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

Dart consumers currently resolve the native wrapper's Cargo graph and inherit build paths from each machine. That makes published-package builds sensitive to local dependency resolution and prevents byte-for-byte reproducibility.

This PR:

• tracks the production native/Cargo.lock and builds it with --locked;
• keeps the workspace path overlay local-only and explicitly opts test helpers into development tests;
• remaps package, output, Cargo, and Rustup paths in the Dart native build hook;
• adds focused flag and artifact-check tests;
• documents reproducibility and isolated consumer smoke-test procedures.

Deliberate non-goals:

• no changes to the Dart public API or generated bindings;
• no changes to the production feature set beyond making its dependency graph explicit and locked;
• no full reproducible Android build in this repository.

[coveralls]

Coverage Report for CI Build 32084577387

Coverage remained the same at 86.718%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

---

Coverage Stats

Relevant Lines:	16458
Covered Lines:	14272
Line Coverage:	86.72%
Coverage Strength:	343.5 hits per line

---

💛 - Coveralls