test(db): document the range count contract's dedicated stack

[xav-db]

Main now includes the range-intersection count contract's 16 MiB stack fix through #1172. Merge current main to resolve the duplicate change and retain the explanation that the debug future must be constructed and polled on the dedicated thread. The remaining diff is two comment lines; the fixture and every assertion match main.

libtest → dedicated 16 MiB thread → Tokio → unchanged contract

Validated the merged branch on fresh ARM64 EC2:

• All 58 production contracts passed, plus the count contract with a 1 MiB outer test stack.
• Workspace Clippy (-D warnings) and formatting passed.
• Reviewed the diff and confirmed it changes only the explanatory comment.

The original fix was also validated with rebuilt-image packaging, native-volume persistence, S3/Compose suites, full workspace tests and doctests before main incorporated it.

[matthewsanetra]

Same fix as #1171. Workspace tests and DB all targets pass; the two red jobs are time-limit cancellations (Clippy after every test step passed; DB coverage, which times out on every PR since the async index queue).

[matthewsanetra]

Superseded: main already has the same change (92c1575, via #1172), and #1173 raised the quality job's time limit. Thanks for the Linux validation; it matched ours.