* docs(#1609): design note — verifier reach = spec reach
Adds docs/design/verifier-reach.md (new docs/design/ dir): a design-rationale note recording the probe family's organizing principle — a goal-backward verifier only checks assertions that exist, so reliability comes from widening the spec (edge + prohibition probes), not sharpening the verifier. Expands the rationale already stated in ADR-857's verification-substrate section and the CONTEXT.md PROBE.principle predicate. Author's calibration numbers are hedged as internal research, not GSD claims.
Closes#1609.
* docs(#1609): split out plan→execute generalization to keep note within #1609 scope
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