diffguard-analytics: 5 more public functions missing #[must_use]

[EffortlessSteven]

Location\n — multiple functions after the ones already filed in other issues:\n\n| Line | Function |\n|------|----------|\n| 139 | |\n| 203 | |\n| 211 | |\n| 233 | |\n| 253 | |\n\n## Issue\nThese public functions return values that callers may silently discard. The attribute is already applied to (line 100, also in this file) and many other functions across the codebase, but these 5 were missed.\n\nClippy confirms: warning.\n\n## Priority\nLow — silent discard of trend/fingerprint results would be a logic bug but not a correctness issue.\n