Add public.zig as the libpanto root; delete root.zig

public.zig is now the sole exported root (build.zig points the panto module
at it). It defines the curated public API allowlist: behavioral facades
(Agent/Stream/Conversation wrapping a pointer to the heap-pinned internal),
the ResultParts struct, Pricing, the session seam, and data-type aliases
for the Config/Conversation/Event families.

A clearly-marked transitional block re-exports the internal module
namespaces and the freestanding result-part helpers the panto CLI still
imports directly; Phase 4 trims these as the CLI migrates onto the curated
surface. Stream.Phase is now pub so State can alias it. The refAllDecls
test contract moved from root.zig into public.zig.

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