File provider API: traversal and discovery #460

Epic: #444

The file provider's tree-walking and matching surface, shaped for Starlark consumption rather than for
the Go call site that grew it.

Spec: 3.5.4-file-provider.md

Moved 2026-09-07 from Epic: ResourceModel to Epic: Providers (#815): the resource model holds the framework — value types, sealed resources, coverage discipline — and this is one provider's surface, beside pkg, file, lint, plan and shell.

Epic: #815