Composable Model Graph library

Name the root package `modelgraph` (was a confusing `composable-model-graph`
leftover that collided with the sibling repo) and expose the built core via
root `exports`/`main`/`types` pointing at `packages/core/dist`. Add a
`prepare` script (`tsc -b packages/core`) so installing this repo as a
GitHub-URL dependency builds core's dist on install without publishing to npm.

Co-authored-by: Cursor <cursoragent@cursor.com>

Transforms can now be connected as a branch/merge graph via an optional
`connections` list, while a transforms-only config still runs as a straight
line (full back-compat). A transform runs once every transform feeding it has
produced output; transforms whose inputs are all ready run together, and a
merge transform receives the ordered list of its feeders' outputs. Cycles are
rejected; repeating a run stays a concern for the layer above the graph.

Co-authored-by: Cursor <cursoragent@cursor.com>