docs: cut 0.9.18 changelog and document the incomplete-build refusal (--allow-partial)

Stamp the 0.9.18 release date and add a README troubleshooting entry for the
new #1951 behavior: an incomplete extraction refuses to overwrite a larger
existing graph, with --allow-partial as the escape hatch.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 Full release notes with details on each version: [GitHub Releases](https://github.com/safishamsi/graphify/releases)
 
-## 0.9.18 (unreleased)
+## 0.9.18 (2026-07-17)
 
 - Fix: an incomplete extraction no longer force-writes a partial graph over a complete one (#1951, thanks @TPAteeq). A crashed AST/semantic pass, a some-chunks-failed run, or a walk that couldn't fully enumerate the corpus (permission-denied subtree) produced a smaller graph that the `to_json(force=True)` path wrote anyway, bypassing the #479 shrink guard; the `--no-cluster` raw dump had no guard at all. Both paths now refuse to overwrite a larger existing graph when the run was incomplete (exit 1, nothing written) unless `--allow-partial` is passed, and a present-but-unparseable existing graph fails closed (a corrupt/mid-write file could be hiding a complete graph). `detect()`'s `walk_errors` now count as incomplete too.
 - Fix: `graph.json`, `manifest.json`, and the other JSON artifacts are now written atomically (#1952, thanks @TPAteeq). A kill, OOM, or ENOSPC mid-write used to leave a truncated file — and a truncated `graph.json` then wedged every later run via the shrink guard's fail-safe. Writes now go to a temp file in the same directory and `os.replace` into place (writing *through* a symlink so shared-store setups keep working); the writers the original change missed (the `--no-cluster` dump, `merge-graphs`/`merge-chunks`/`merge-semantic`, the analysis/labels sidecars, and the global graph/manifest) are routed through it too.

README.md

@@ -553,6 +553,9 @@ PowerShell treats a leading `/` as a path separator. Use `graphify .` (no slash)
 **Graph has fewer nodes after `--update` or rebuild**
 If a refactor deleted files, the old nodes linger. Pass `--force` (or set `GRAPHIFY_FORCE=1`) to overwrite even when the rebuild has fewer nodes.
 
+**`extract` exits with "extraction was incomplete ... refusing to overwrite"**
+When an extraction pass crashes or a walk can't fully read the corpus, the run would be smaller than a complete one, so `graphify extract` refuses to overwrite a larger existing graph with the partial result (protecting your `graph.json`). Fix the underlying failure and re-run, or pass `--allow-partial` to overwrite anyway.
+
 **Graph has duplicate nodes for the same entity (ghost duplicates)**
 Ghost duplicates (same symbol appearing twice — once from AST extraction with a source location, once from semantic extraction without) are now automatically merged at build time. If you see this in a graph built before v0.8.33, run a full re-extract to clean up:
 ```bash