PlainModelDocument class final

A ModelDocument assembled by hand, from plain lists.

The one fake every writer's test needs, so there is one of it. A test of a writer wants a document that came from nowhere in particular — no decoder, no file — and before this each writer's test file declared its own private _Document doing exactly that, field for field the same as the next one's. Two copies of a class are two places a third field can be added to one and forgotten in the other.

nodes defaults to empty rather than to ModelDocument's own "one node per surface" fallback, so a document built here has exactly the fields it was given and nothing a later change to that fallback could move underneath a test that never asked for it. A test of the fallback itself constructs ModelDocument some other way.

A second, real use: rebuilding a document with one field changed. flutter3d_build's converter (ap-07) reads every field off a decoded document and rebuilds one with images alone re-encoded to a compressed format — every other decoder-specific override (a real node hierarchy, a skin) already resolved to a plain value by the time it is read, so copying the values rather than wrapping the original loses nothing and needs no decorator that forwards every current and future getter.