D759 — concept_expand leaves a dangling asset link as written

Decision. When concept_expand rebases the concept's relative links (D295), a link to a non-Markdown file (an extension other than .md) whose target does not exist is left exactly as written. After the move it resolves inside the concept's own directory. Links to existing files and to concepts (.md or no extension, dangling or not) are rebased as before.

Why. The natural order for a dossier with scripts or images is: write the body, expand, then asset_write the files it links. Rebasing kept the old target (map/build.py), so every link broke once the assets landed next to index.md (#759). A non-Markdown file can only legitimately live inside an expanded concept — in a map directory it is a stray_file — so for a dangling one the concept's own directory is the only target that can ever be valid. Rebasing preserves what a link resolves to; a dangling link resolves to nothing, so there is nothing to preserve.

Alternatives rejected. - expanded: true on concept_write / concept_new: one more parameter on two tools of the default profile for a case the rebase rule now covers, and it would not fix a concept that was written plain and expanded later. - Leave every dangling link untouched, .md included: a dangling concept link most likely names a sibling concept still to be written in the same map; pointing it inside the dossier would break it differently.

Consequences. The exemption is decided against the working tree at expand time. concept_move and concept_merge keep the plain D295 rebase: their target directories are not the concept's own asset home.