test: add EAGER/LAZY mode relation coverage

[membphis]

Summary

• Add FFI-level EAGER/LAZY mode relationship tests for acceptance parity, deferred lazy access errors, depth limits, max_depth boundaries, trailing-content behavior, and root/cursor consistency.
• Fix skip-cache publication so malformed lazy child walks cannot leave a partially populated cache that turns later parse errors into QJSON_NOT_FOUND.
• Document the lazy nested-container offset boundary where root path resolution has no stable inner offset but cursor-relative access can report the container opener.

Fixes #136

Test Plan

• make lint
• cargo test --release --test ffi_mode_relations
• cargo test --release --no-default-features --test ffi_mode_relations
• cargo test --release
• cargo test --release --no-default-features
• cargo test --features test-panic --release
• cargo test --no-default-features
• LuaJIT busted via LuaRocks 5.1 runner: 257 successes / 0 failures / 0 errors / 3 pending
• Lua lint via LuaRocks 5.1 runner: 0 warnings / 0 errors

Note: local make test reached the Lua step and then failed because the default Homebrew busted wrapper targets Lua 5.5. The Lua tests were rerun successfully with the installed LuaRocks 5.1/LuaJIT runner, matching the CI ABI path.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 2ca7f41d-6928-4cfd-b352-32a77e247926

📥 Commits

Reviewing files that changed from the base of the PR and between 6225ce2 and 4cff06d.

📒 Files selected for processing (3)

• src/cursor.rs
• src/skip_cache.rs
• tests/ffi_mode_relations.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/issue-136-mode-relations

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