fix(assets): sample latest owner routes without blocking

[P0m32Kun]

Summary

• keep the compact owner-route budget at 16 while selecting the newest observable filesystem mtimes
• expose collection time, timestamp coverage, and per-route modifiedAt evidence
• separate intentional route sampling from evidence-loss truncation so normal reviews do not fail on asset count alone
• bound filesystem metadata probes to 32 concurrent operations

Traceability

Implements docs/specs/2026-08-03-asset-route-selection-freshness.md (AC-1 through AC-5).

Validation

• npm run check on Node 24: 1020/1020 tests passed
• npm and runtime package verification passed
• focused doc-link graph tests passed 6/6

Risk

Filesystem mtimes can be absent or coarse. The envelope reports timestamped and untimestamped counts and uses deterministic fallback ordering instead of inferring freshness.