### What?
Makes Turbopack import tracing recognize the module graph's explicit
entries as roots, including when an entry participates in an import
cycle. Adds focused graph-level coverage and a Next.js production
fixture that exercises issue formatting through a cyclic graph.
If a malformed graph still has no path to any explicit entry, tracing
now returns a minimal trace and emits a bug-severity issue instead of
panicking.
### Why?
The import tracer inferred roots from nodes with no incoming edges. A
valid cycle that points back to an entry gives every node an incoming
edge, so the tracer could not find a root and panicked while formatting
another diagnostic. The graph already records its entries explicitly,
making them the authoritative and cycle-safe definition of a root.
The same topology-based assumption affected entry membership checks, so
those now use the explicit entry list as well.
### How?
- Resolve import-trace paths against node indices derived from
`GraphEntries`.
- Preserve a defensive fallback for malformed graphs and classify that
invariant violation as an implementation bug.
- Cover cyclic entry paths, malformed rootless graphs, absent paths, and
the product-level issue-formatting path.
Fixes#98205
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Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
Tobias Koppers
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