docs: require screenshots, recording, category and data licence on new-layer PRs (#957)

Document visual evidence, category and data licensing requirements for new layers, using the changelog fragment workflow.

Contribution by @asturksever.

.github/pull_request_template.md

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+## What this changes
+
+<!-- One or two sentences: what changed and why. Link the issue if there is one. -->
+
+## How I verified it
+
+- [ ] `npm run build`
+- [ ] `npm test`
+- [ ] `npm run test:track` (dev server running)
+- [ ] Feature gate for the area I touched: <!-- e.g. `node scripts/qa-radio.mjs`, or "none applies" -->
+
+<!-- Screenshots are welcome for anything visual. -->
+
+## New layer checklist
+
+<!--
+Fill this in if the PR adds a new data layer. Delete the whole section if it doesn't.
+See "Adding a new layer" in CONTRIBUTING.md.
+-->
+
+**Screenshots**
+
+<!-- At least one screenshot of the layer on the globe, plus its info card or panel if it has one. -->
+
+**Recording**
+
+<!--
+Preferably 15 to 30 seconds: turn the layer on, move around, open a record.
+Drag an .mp4 or .mov in here; GitHub accepts up to 10 MB on free accounts.
+If it's bigger, link to it instead.
+-->
+
+**Proposed category**
+
+<!--
+Which layer-panel group should it sit in? The current groups are in PANEL_GROUPS
+in src/ui/layerPanel.js: Movement, Cameras, Infrastructure, Events, Weather, Utilities.
+If none fits, propose a new group and say why.
+-->
+
+Category:
+
+**Data source and licence**
+
+- Source (provider and endpoint):
+- Licence or terms (link):
+- Attribution shown in the app:
+- Needs a key? (none / optional / required):
+- Fetched at runtime, or bundled in the repo:
+
+<!-- Bundled data must be data we have the right to redistribute. -->
+
+- [ ] Added a row to `DATA_SOURCES.md`
+- [ ] Credit is registered in `src/data/dataCredits.js`
+- [ ] Share-link token assigned with `npm run layer-token:next`, if the layer is shareable
+
+## Docs
+
+- [ ] `docs/CURRENT-STATE.md` updated and a `changelog.d/<pr-number>-<slug>.md` fragment added, or this PR doesn't change runtime behaviour