Alternatives and stance

Adds the claims a paper argues against. Each is an ordinary claim file — prefixed alt- — whose assertions entry carries stance: rejects rather than asserts, so an alternative the paper rules out is a node in the graph like any other and an eliminative edge has something correct to point at.

Given the tree's controls and empirical claims — slug, role, panel and sentence — the paper's questions, and the Results prose with span ids, it returns the rivals the paper raises to reject (or entertains and leaves open) and the controls that eliminate each. It writes an alt- claim file per rival — the stance on the assertion, addresses linking the question it answers where index.md states questions, the body carrying the why — and adds the rules-out edges from the named controls through the same validator every edge goes through, so a rules-out whose target is asserted, or whose source is not a control or empirical claim, is dropped with a logged reason. It is idempotent: an alternative already present by slug is updated, not duplicated.

--dump-prompt writes the exact request and exits, and --answer feeds a reply back through the same validation, so a model with no backend here can still answer it.

Before these existed, a rejected alternative lived only as prose inside the claim that dismissed it, and an edge that should have pointed at it pointed at whatever claim was nearest.