test(frontseat-pipeline): pin the export-registry contract for image entities

[joaodinissf]

A contract test, no behaviour change. Green on main.

An entity root can hold both a language manifest and an image manifest — a Go service with an apko.yaml beside it — and that shape has to reach a plan. It is the ordinary way to ship a service as a container, and the registry is built to allow it: named exports stack, and exactly one plugin declares the unnamed default.

Both halves of that contract have been broken in turn:

• a plugin claiming no default leaves the primary ambiguous (failed to build action graph: build export registry: ambiguous primary export on entity #131, Image plugins never claim the default export, so #131 still reproduces (gib + apko) #199)
• a plugin claiming it unconditionally collides with the language plugin that already owns it

The second is the tempting fix for the first, which is why this is a test rather than a comment. It fails with duplicate export "" against that change and passes on main — verified both directions.

Two things the tests also pin, both of which I got wrong before running them:

• The ambiguity is cross-plugin. One plugin may stack its own named exports; the rejection only fires when two different plugins each supply a named export and neither declares a default.
• A shared export name trips the duplicate check first, so it never reaches the ambiguity path at all.

Opened separately from #240 so the contract lands regardless of how that PR is resolved. If #271 replaces the export registry, this goes red and has to be consciously ported — which is the point: a bullet in an issue can be skimmed, a red test cannot.

This pull request was published with assistance from Claude.