importdemo/v0 package

Package importdemo shows a realm importing and composing packages from ANOTHER namespace — the whole point of the demo.

v2 imported gno.land/p/archive/dom, which is published on no chain we target (absent on both sapphire and pearl), so it type-checked locally but could never be deployed: `addpkg` failed with `could not import gno.land/p/archive/dom (unknown import path)`. v2 is archived (`ignore = true`); this version rebuilds the same thread-of-posts idea on foreign packages that are actually on chain — `p/nt/avl/v0` for ordered storage and `p/nt/ufmt/v0` for formatting.

Deliberately keeps no realm state: Render builds the thread on each call, so output depends only on the code and the demo is reproducible.