test: extend max-depth stress coverage (#140)

[membphis]

Closes #140.

What

Adds deterministic max-depth boundary tests that pin the named limits the fuzz_depth target only reaches probabilistically. As noted in the issue review, roughly 70% of the acceptance criteria were already covered (#142 mode relations, fuzz_depth, ffi_parse_error_offsets, options.rs unit tests, options_spec.lua), so this PR is scoped to the genuine deterministic gaps and does not re-cover EAGER/LAZY equivalence.

tests/ffi_depth_stress.rs (new)

• default max_depth = 0 -> 1024 resolved at the FFI/parse level: accept 1024, reject 1025 with offset 1024
• 4096 ceiling: accept 4096, reject 4097
• clamp of over-ceiling requests (4097, 5000, u32::MAX) to 4096 — verifies the larger request is not honored
• object-only and mixed nesting boundaries (deterministic coverage was array-only)
• pathological 100k-deep input rejects fast as QJSON_NESTING_TOO_DEEP without recursing
• cursor reachability of an accepted at-limit (1024-deep) document down to the innermost scalar

All cases run in both EAGER and LAZY modes.

tests/lua/options_spec.lua

• propagate default-depth nesting error (1025) in both default and lazy mode
• accept exactly 1024 at the default
• clamp an over-ceiling max_depth to 4096 through the wrapper

without duplicating the existing accept / message / lazy / fractional cases.

Stress placement

The deep stress case is a lightweight deterministic test running in the standard CI gates (cargo test --release and --no-default-features); fuzz_depth continues to provide randomized exploration. No separate scheduled workflow is needed. EAGER/LAZY depth equivalence is already fixed by #142 and is intentionally not re-covered.

Verification

• cargo test --release — 168 + all integration tests pass
• cargo test --release --no-default-features --test ffi_depth_stress — pass (scalar parity)
• cargo clippy --release --all-targets -- -D warnings — clean
• Lua busted options_spec.lua — 12/12 pass

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 777219ff-9b86-4255-a59a-e638890ee835

📥 Commits

Reviewing files that changed from the base of the PR and between 58f10eb and dc8debe.

📒 Files selected for processing (2)

• tests/ffi_depth_stress.rs
• tests/lua/options_spec.lua

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/max-depth-stress-140

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