Adversarial review findings:

1. P2 pkg/synth/check.go: CheckOptions.Assertions exposes the full CheckThresholds type, including MaxDepth, MaxFanOut, and MaxSpans, but Check() only uses Assertions for percentile checks. The CLI copies static thresholds into legacy fields before calling Check, so the command works, but package callers who load a checks file and pass CheckOptions{Assertions: assertions.Checks} will not get the same behavior. Worse, they will still hit zero-valued MaxDepth/MaxFanOut/MaxSpans unless they also know to copy those fields manually. Either make Check() apply static assertion fields too, or do not expose static thresholds through CheckOptions.Assertions.

2. P3 pkg/synth/check.go: percentile assertion results drop the selected scenario's Distribution. The CLI therefore prints FAIL p99-depth: ... but not the p50/p95/p99/max line for the scenario that actually caused that percentile assertion. This is especially confusing when the static max-depth result and the percentile assertion choose different scenario sets. Set Distribution: &bestDist on these results so the existing output path prints the supporting distribution.

3. P3 docs/man/man1/motel.1: the man page is now stale for motel check; it does not include --checks. It was already missing some newer check flags, but this PR adds another user-facing flag and updates only docs/reference/synth.md. If the man page is shipped, this should be updated or deliberately marked/generated differently.

Open question: should --checks accept URLs officially? LoadCheckAssertions uses the shared source reader, so URLs work, but the flag help says "file". Either document URL support or restrict it.

Review scope: reviewed the PR diff against main, checked the published PR body, inspected surrounding check/scenario code, searched for stale CLI docs, and hand-ran a failing percentile check case.