[dsl] Sub-scenario invocation in do blocks is not lowered — the DSL's reuse mechanism does not execute (§7.6.2)

[JArmandoAnaya]

What

Invoking a scenario (rather than a §8.8 movement action) inside a do block is not lowered — and the DSL's primary reuse mechanism therefore does not execute:

• frontends/dsl/src/lower.cpp:454-462 — a non-§8.8 invocation warns "scenario invocation is p8-s2, [p8-s2] Composition operators: serial, parallel, one_of #45"; [p8-s2] Composition operators: serial, parallel, one_of #45 is closed
• lower.cpp:592-597 — an invocation with no actor prefix is a hard error ("a movement action is invoked on one participant of this scenario (§7.2.2.4.7)")

The DSL coverage matrix implies it works: §7.6.2.1.1 behavior invocation In/In, §7.6.2.2 actor binding In/In, §7.6.2.3 In. It is documented nowhere as out of scope. Found by the post-P8 audit; highest-severity untracked DSL gap.

Sketch

Lower a composite scenario invocation by inline-expanding the callee's do composition into the caller's composition tree (concrete arguments bound per §7.6.2.2; labels/qualified names kept unique; recursion diagnosed). Trace acceptance per §7.6 permits this operational choice; document it in an ADR amendment.

Done when

• Concrete sub-scenario invocations execute (nested do, actor binding, with: modifier blocks on the invocation)
• Red fixtures for the genuinely unsupported forms with §-cited diagnostics
• Matrix rows §7.6.2.1.1/.2/.3 tell the truth; dsl_lowering_test.cpp snapshots added; an XML/DSL golden exercising a sub-scenario considered