ADR 0016 follow-ups: repeated references, whole-file links, an opt-in depth lint

[aslakhellesoy]

Follow-up to #104 (ADR 0016). Smaller open questions, collected — each is a decision rather than a defect, and none blocks anything.

• The same section referenced twice in one example. Works today: the steps run twice. Is that a legitimate "do it again", or an authoring mistake worth a diagnostic? (ADR open question 5.)

• Fragment-less whole-file references ([Setup](./shared/setup.md)). Supported, and the one form whose meaning silently changes when the target file grows a second example. Keep, or require an anchor? (ADR open question 6.)

• An opt-in depth lint. Nesting depth is deliberately a style question the parser doesn't enforce (see explanation/reuse.md), but reference/lint.md is where checkable house style already lives. A rule that is off by default and warns past a configured depth would let a team set its own ceiling without Varar picking one — or it's the same prohibition wearing a hat. (ADR open question 9.)

• A referenced section's own scopeStack is discarded in favour of the referring example's; the heading serves only as the anchor and the link text. This is what ships, and the 21-reference-consumed bundle pins it — confirm it is intended, and say so in the reference. (ADR open question 4, effectively answered by implementation.)

[aslakhellesoy]

Resolved by #113 (merged as d5fde84): see the ADR 0016 section "Open questions, and how each was decided" and reference/examples.mdx for the decision as documented.