docs(design): canonical shippable strategy form is a Composite blueprint (C20)

Add a C20 realization recording the resolution of the GER40 deep-dive's #94
finding: a hand-wired FlatGraph is not a shippable strategy (no param_space, so
World-opaque); the canonical form is the Composite blueprint, the FlatGraph its
compiled substrate (C23). Pins the bar-period-as-structural-construction-arg
decision (spec 0051) to the structural-axis-vs-tuning-param split.

refs #94

docs/design/INDEX.md

@@ -898,6 +898,21 @@ Node/graph abstraction (C9) and makes "10 strategies in one environment" and
 authoring (not config) preserves full programmatic power — conditional/adaptive
 matrices, generated axes, custom wiring — and avoids re-introducing the DSL trap
 C17 rejects.
+**Realization (GER40 session-breakout blueprint milestone, 2026-06-17 — refs
+#94/#96/#97, spec 0051).** Made concrete on the first real-data strategy: a
+**hand-wired `FlatGraph` is not a shippable strategy** — it carries no
+`param_space()`, so the World families (sweep / walk_forward / compare, C21)
+cannot consume it without a hand re-author (the friction the GER40 deep-dive
+fieldtest surfaced). The **canonical shippable form is the `Composite`
+blueprint** (the authoring/source level, C9/C19); the `FlatGraph` is only its
+compiled substrate (C23). The breakout now ships as
+`ger40_breakout_blueprint(bar_period, …)` whose `param_space()` is exactly its
+tuning knobs (`{entry_bar.target, exit_bar.target}`); the **bar period is a
+construction argument** binding Resample + Session together — a structural
+matrix axis (C12: a different period is a *different strategy*, not a sweep
+point), never a `param_space` entry, so a sweep cannot desync the two clocks.
+This is the concrete instance of the structural-axis-vs-tuning-param split
+above (and `delay.lag`, a C8 structural constant, is bound out of the space).
 
 ### C21 — The World: the meta-level is the product
 **Guarantee.** Above the harness sits the **World** — the project's program /