Add cross-platform boundary tests for the environment API

## Summary

The new environment API needs a compact cross-platform regression set that defines its boundary behavior independently of individual platform providers.

## Related issues

- #423
- #424

## Scope

Add focused Wave corpus cases for environment lookup, presence checks, typed integer parsing, default values, invalid names, missing keys, empty values, destination capacity, and numeric boundaries. Reuse deterministic fixture variables supplied by the test runner instead of relying on a developer machine's ambient environment.

## Acceptance criteria

- Tests cover missing, empty, one-byte, and multi-byte values.
- Tests distinguish “not found” from “destination too small”.
- Tests cover valid and invalid signed i32/i64 values and both numeric boundaries.
- Test metadata injects deterministic environment variables where required.
- Cases run on every native CI platform whose environment provider is implemented.
- Platform stubs are skipped or classified explicitly rather than silently counted as passing.
- The suite is documented briefly next to the test cases.

Start by locating the existing Wave corpus cases for the environment API and the test-runner metadata that supplies fixture variables. Extend those cases to cover the listed value, lookup, capacity, and numeric-boundary behaviors, then run them across each implemented native CI platform and verify stubs are explicitly skipped or classified and the nearby suite documentation is present.

Written by the indexing model from the issue text.