Contribution point resolution at runtime, and the NavBar widget

[einari]

Context

Screenplay's contribute to <Name> construct (see Screenplay issue
"contribution points and navigation") needs a runtime implementation:
collecting contributions from across the module/feature tree into the
widget that declared the contribution point, using nearest-enclosing
resolution.

Proposal

• Implement the aggregation as a computed view, analogous to how a
  projection produces a read model — the NavBar (or any widget declaring
  contributes X) doesn't get wired to a fixed source, it renders
  whatever is currently contributed from the tree beneath it.
• Implement nearest-enclosing resolution: walk outward from a
  contribution's declaration site to the first enclosing template that
  declares a matching contribution point, same algorithm as existing
  inside-out name resolution, just inverted (push vs. pull).
• NavBar widget specifically: consumes Navigation contributions and
  owns how a contribution's navigate to <Screen> by <param> becomes a
  concrete route — URL path, query string, or native deep link — as a
  rendering concern, not a modeling one. Should be configurable per
  renderer (Scene.React vs. a future native renderer) without changing
  the contribution's declared shape.
• Support ordering (order property on a contribution) affecting render
  order in the consuming widget.

Open questions

• Ambiguous contribution resolution (two equally-near matching points) —
  confirm this surfaces the same warning UX as ambiguous query names, and
  decide whether Studio needs a design-time-only strict mode that
  upgrades these to errors.

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Dependencies

Depends on

• Contributes and contribute to Screenplay#92 — the contribute to / contributes construct this implements
• Scene object model, and split into Model / Engine / React #1 — the object model the aggregation is computed over

Blocks

• UI starters (kits) and their sample gallery #6 — the gallery's nav shell comes for free from this mechanism
• Navigation / contribution designer UI StudioIssues#158 — navigation / contribution designer UI

Transferred from Cratis/Stage — the runtime and NavBar widget concerns here belong with Scene's engine, next to the other resolution engines. Cratis/Stage#37 remains the Screenplay → Scene seam.

Part of the screen work — build order and full dependency map: #7

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Implementation notes

Lands in Scene.Engine (TypeScript) — the aggregation is computed over the object model at runtime in the browser. The NavBar widget's route rendering lands in Scene.React, per the per-renderer configurability the proposal calls for.

The contribution shape itself (what a contribution point declares it needs) is part of the C# Scene.Model and its TypeScript mirror — see the dual-stack decision in #1.

[einari]

Fixed in #11.