feat(nanoviews): expose swap_, the block if_ and switch_ are built on

[dangreen]

if_ and switch_ were both built on an internal block that renders whatever a decider returns for a value, and rebuilds it when the value changes. That block is useful on its own, so it becomes public as swap_.

tbody()(
  swap_($tab, tab => (tab === 'list' ? List() : Grid()))
)

Why not dynamic

This is the job other libraries hand to <Dynamic component={X} {...props}/> or <component :is>, and the todo entry that started this called it exactly that. But those exist because in JSX and in templates the tag position is not an expression — you cannot write "call this function with these props", so a component has to be smuggled in as a prop and the rest spread over it. There is no tag position here: a component is a plain function, and Comp(props) already is what <Dynamic> is for. A wrapper with that signature would re-implement function application, and it would be weaker — with a decider the props are built per value, inside the closure, which one props object cannot do.

So swap_ is the general block and Dynamic is one line of it. Naming it after the narrow case would have promised a signature the library does not need.

Why swap_

The name carries the thing consumers get wrong. A binding updates content in place; this block replaces it — the old subtree is destroyed and everything it held goes with it. decide said a decision happens and stayed silent about that, which is the half people learn by losing state.

The trailing _ follows the flow vocabulary — if_, for_, switch_, case_, each_ — where it marks the DSL rather than dodging a keyword.

What moved

• internals/flow/decide.ts → internals/flow/swap.ts; reactiveDecide → swap, and the parameters lose the conditional vocabulary ($condition/decider → $value/render) now that the block is not only about conditions.
• The value-or-accessor entry moves out to the public flow/swap.ts as swap_; if_ and switch_ call it.
• A spec for the public function: a static value renders once without subscribing, and a changed value gets a new node rather than a reused one — the property the name is chosen for.

Cost: 10 B gzip on the all-publics bundle for the new export. Average usage is unchanged; nothing that does not import swap_ pays for it.

One correction rides along, in the specs of the directory this touches: the flow specs called their group logic, which has never matched the directory.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.29%. Comparing base (18127c1) to head (6440225).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #203   +/-   ##
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  Coverage   85.29%   85.29%           
=======================================
  Files         140      141    +1     
  Lines        3168     3168           
  Branches      593      593           
=======================================
  Hits         2702     2702           
  Misses        335      335           
  Partials      131      131           

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