A durable system message may contain multiple text blocks or other content. The runtime-context helper recognizes only one text block; treating an unrecognized system message as empty allowed clearing and incapable-route normalization to leave its instructions active. Keep undefined text distinct from genuinely empty content so every noncanonical node is replaced or cleared instead of silently skipped. Runtime-context snapshot equality remains unchanged.
Regression: a multiblock head and nontext tail previously survived a clear request; the new source-level case fails before this fix. Both system-prompt projection and actual-loop admission suites pass (21 tests).