Admit Core 0.3.0 on ordinary mapped-source runs (prerequisite for the 0.3.0 recording set, #174) #188

Gap

The Runner admits Core 0.3.0 only on the selected retained-column path, which runner v0.26.0 pins to Spec v0.3.0. Ordinary mapped-source runs still evaluate under 0.2.0-draft, even though Runtime v0.39.0 evaluates Core 0.3.0.

This blocks #174, the genuine 0.3.0 dialect recording set. All 47 of its scenarios are ordinary mapped-source runs, so they cannot be produced at 0.3.0 until ordinary admission exists. The v0.26.0 final prep (#187) established this.

Required behaviour

• Ordinary runs admit packs that declare specVersion 0.3.0, evaluated through the pinned Runtime release that supports it. 0.2.0-draft packs keep working unchanged.
• Admission names the exact Core version each run used, in the run record, as it does today for the selected path.
• Version selection is explicit. A pack's declared specVersion selects the evaluator. There is no silent upgrade of a 0.2.0-draft pack, and an unsupported version is refused by name.
• Tests cover both versions side by side, the refusal of an unsupported version, and records that name the version.

Once this lands, #174 can generate the 0.3.0 set through the Runner's own producer.