Trim duplicate overlay tests

[yvette-carlisle]

Summary

Builds on the landed behavior-domain test reorganization and does a stricter second pass over duplicate overlay tests. This keeps production behavior unchanged and consolidates same-entry/same-branch/same-observable-result cases into table-driven tests.

• reduce the workspace nextest inventory from 600 tests after the first pass to 564 tests, with 570 total nextest entries including 6 bench entries
• consolidate repeated overlay runtime/rendering assertions across scroll input, toolbar, export, capture affordances, selection, self-capture, stream refresh, and live runtime tests
• remove only tests that were strict duplicates, implementation-detail assertions, or already covered by an equivalent visible contract

Validation

• cargo +nightly fmt --all
• cargo nextest run -p rsnap-overlay --lib overlay::tests
• cargo vstyle curate --workspace --all-features
• cargo make lint-rust
• cargo make checks

Notes

No production code changes. The remaining larger files still map to real behavior surfaces rather than historical catch-all buckets.