docs(app-developer-guide): add the missing local manifest-validation step

The guide walked a developer from manifest to install but never told them how
to validate the manifest locally first — despite scripts/validate-app-manifest.sh
existing for exactly that. A developer's first signal that their manifest was
wrong would have been an install failure on a node.

Adds a "Validate Your Manifest" step at the top of Testing, pointing at the
script (recently fixed — it had been rejecting every manifest because it shelled
out to a missing ruby). Notes the strict behaviour a new submitter hits, e.g.
an unpinned :latest tag is rejected, and that the Rust parser is canonical.

Verified: the install RPC example in this guide (id + dockerImage) matches the
handler; the cargo test target crate name (archipelago-container) is correct.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/app-developer-guide.md

@@ -341,6 +341,21 @@ cmp -s app-catalog/catalog.json neode-ui/public/catalog.json
 
 ## Testing Your App
 
+### Validate Your Manifest
+
+Before anything else, check your manifest against the schema and the app-submission
+rules:
+
+```bash
+./scripts/validate-app-manifest.sh apps/my-app/manifest.yml
+```
+
+It reports `STATUS: APPROVED` or `STATUS: REJECTED` with the specific failures —
+including the ones that block submission, such as an unpinned `:latest` image
+tag (new apps must pin a concrete version). It needs `python3` and PyYAML; it
+tells you if either is missing. The Rust parser in `core/container/src/manifest.rs`
+remains the canonical validator — this script is the fast local preflight.
+
 ### Local Testing
 
 ```bash