docs: add ADR-0001 for self-contained share links

docs/adr/0001-self-contained-share-links.md

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+# Self-contained share links
+
+Status: Accepted
+
+Sharing a Radar must work with **no backend** — the app is a static SPA deployed to
+nginx (Coolify) and GitHub Pages. We encode the entire Radar as `base64url(JSON(payload))`
+in a `/share?d=…` query parameter rather than storing it server-side behind a short id;
+the importing client validates and clamps the untrusted payload (`useRadars.importRadar`)
+before it ever touches the store.
+
+## Considered options
+
+- **Self-contained link (chosen)** — full Radar in the URL.
+- **Server-stored short link** — POST the Radar, share a short id. Rejected: requires a
+  backend + datastore, which the zero-infra constraint forbids.
+- **Compressed payload (LZ-string)** — shorter URLs, same self-contained property.
+  Deferred, not rejected: the §7 fallback if link length becomes a problem.
+
+## Consequences
+
+- **Got:** zero infrastructure; links never expire; works on any static host; offline-capable.
+- **Paid:** long URLs (no compression today); the full Radar is exposed in the URL (no
+  privacy boundary); no revoke and no edit-after-share — a shared link is a frozen copy.
+- Mitigated by the cardinality caps (≤7 Criteria, ≤5 Profiles) which bound payload size,
+  keeping URLs within browser limits at current scale.