Visualizer excludes index.md from the graph but not log.md

SPEC.md §3.1 reserves both `index.md` and `log.md` as non-concept files, but the viewer's graph generation only special-cases `index.md` (`_INDEX_NAME`, `generator.py:72`); a spec-conformant `log.md` shows up as a bogus graph node (type `Unknown` when it has no frontmatter). The sample bundles ship no `log.md`, so this doesn't surface in-repo. (Observed live with a real bundle: a type-`Unknown` node with id `log`.)

**Suggested fix.** Exclude `log.md` alongside `index.md` in `_walk_concepts`.

Read SPEC.md §3.1 and inspect _walk_concepts in generator.py around line 72, where index.md is excluded. Update the graph-generation exclusion so a spec-conformant log.md does not become a node, then verify that the generated graph contains neither reserved file.

Written by the indexing model from the issue text.