Multi-select view (`showMultiSelect()`)

[brillout]

See:

• Preset: research #331

[suleimansh]

Before I build this, two quick confirms:

1. Name: your title says showMultiSelect(), but Preset: research #331 body says sendMultiSelect(). Going with showMultiSelect() to match the showMarkdown() family in System prompt #326. OK?

2. I plan to build it by extending the existing single-select choice gate (the one behind [The Framework] Interactive <Choices> panel + autopilot auto-accept #304 plan-approval): same panel + POST-back resolver, just a checklist that pre-checks the defaults and returns the selected subset, rather than a whole separate gate. Good with that?

Building #330 (param substitution) first since it is independent, then this.

[brillout]

1. Yes, send is a typo, show is correct
2. Sounds good