feat: a third navigation target, for a diagram whose file nobody wrote

[endrix]

Stacked on #37. Review that one first; this branch contains only the second commit.

cal:referencedUri answers two questions at once — drill into this element and show me its source — and that works for exactly as long as both have the same answer.

For a diagram generated from something else they come apart:

• drilling has to stay inside the generated file, because that is where the definitions being drilled into live;
• "show me the source" wants the file a person actually typed.

One slot means picking which of the two features to break. A consumer that hit this picked drill-down and lost source navigation entirely — every "Go to Source" landed in a machine-generated file, or, when pointed at the authored one instead, every element double-click became an attempt to open a source file as a diagram.

So a producer that knows both files can now say both: meta.authoredSource becomes its own pair of keys (cal:authoredUri, cal:authoredSourceRange) and its own context-menu item, Go to Authored Source. The existing two targets are untouched.

Neutrality

Nothing here names a language or a toolchain. Any product with a generation step between what someone wrote and what the diagram is built from has exactly this shape; the platform only learns that such a step can exist. 4/4 gates pass.

Two deliberate refusals

No fallback. The other two targets fall through to the diagram's own file, because there a missing range still means "somewhere in this document". Here an absent value means the producer made no such claim, and offering the generated file under a label that says authored misrepresents which file the reader is being sent to.

Both a file and a line are required. A half-populated locator navigates to the top of a file nobody asked for, which is worse than not offering the item at all.

Compatibility

Inert for every existing product: no authoredSource, no keys, no third menu item. The full platform suite passes unchanged.

Verification

• 5 new tests in authored-source.test.ts, including one asserting that an authored file does not become the drill-down target — the property the whole change exists to preserve
• Full platform suite, 4/4 neutrality gates, build