docs: bound params are overridable defaults — C12 amendment, glossary, guide

Task 7 of the bound-override cycle: the design ledger's C12 records the
bound-as-default semantics (axis 1 may name a bound param; identity reads
the authored document; the retired axes-bind-only-open-knobs restriction
was an implementation consequence, not a recorded decision), the glossary's
blueprint entry carries the same sentence, and the authoring-guide's
data-only starter section explains the one-file run+sweep quickstart.

closes #246

docs/design/INDEX.md

@@ -1132,6 +1132,16 @@ Seed-as-input reconciles Monte-Carlo with C1. The "frozen topology" of the
 atomic unit is one harness instance, selected by the harness's **structural
 axes** (C20); the structural experiment matrix is the outer orchestration over
 this dimension, the tuning sweep the inner (C19/C20).
+**Amendment (2026-07-12, #246).** A bound blueprint param is the param's
+**default**: axis 1 (param-sweep) may name a bound param — the family boundary
+re-opens it on the probe and on every member reload, and the axis binds it per
+cell. "Open" means *must be bound by an axis*; "bound" means *default,
+overridable by an axis*. `run`/`mc` still require every param resolved (a truly
+open param still refuses). Identity is untouched: `content_id_of` and
+`topology_hash` read the authored document, never a re-opened probe, and each
+member manifest records its per-cell bindings as before. The restriction this
+amends — axes bind only open knobs — was an implementation consequence of
+`bind()` shrinking the param surface, not a recorded decision.
 
 ### C13 — Hot-reload is authoring-only; deploy is frozen
 **Guarantee.** A node/strategy is authored as a native Rust `cdylib`,