Builds the value-free digest.json runtime-evidence artifact from an in-memory timeline map (the shape Excessibility.TelemetryCapture.Timeline.build_timeline/3 returns).

The digest is safe by construction: every field is built by explicit construction from allowlisted inputs. It never copies an assigns map, a raw query, params, or bind values. If a field cannot be derived safely it is omitted, not guessed. See the design doc's "Privacy allowlist" section.

Crash-isolated: the whole build is wrapped in try/rescue. On failure it returns a digest with capture.status: :failed and the exception message in capture.warnings, but never raises.