v0.15.3

v0.15.3 — doctor self-check multi-source detection (P2 follow-up to v0.15.2)

Same 3-source detection ladder as v0.15.2's primitives fix, applied to
the self-check (was PATH-only, same blind spot). Adds self.on_path_source

• self.reachable envelope fields. Three-state messaging: reachable=false
  warns; reachable-but-not-on-PATH hints (info); on PATH says nothing.

Verdict semantics preserved — envcheck.missing_count > 0 still means
degraded (honest + actionable). Lifecycle-ring 'install → healthy'
claim is satisfied by v0.15.2 + v0.15.3 multi-source detection.

2 new tests, 411 passing total.