docs: principe — le polyfill compense, il n'étend jamais

Règle posée par l'utilisateur : la seule raison d'être du polyfill est de combler
un retard d'implémentation NextGraph ou un bug. Aucune fonctionnalité additionnelle
propre (pas de feature, d'observabilité, d'API de confort qui ne soit pas « NextGraph
le fera nativement plus tard »). Corollaire : une compensation dont le gap n'est PAS
exhibé sur le broker cible est du poids mort, pas du code défensif — à retirer.

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

packages/client/README.md

@@ -20,6 +20,18 @@ const set = useShape(MyShape, scope);   // filtered to what the identity may rea
 await inbox.post(targetInbox, ref);     // deposit (anticipated SDK API)
 ```
 
+## Principle — the polyfill compensates, it never extends
+
+**The polyfill's ONLY reason to exist is to bridge a NextGraph implementation gap
+or a bug.** Every non-SDK surface must map to a capability NextGraph will provide
+natively, and must fall away at that point. The polyfill MUST NOT add functionality
+of its own — no bespoke features, no observability/tooling, no convenience API that
+isn't strictly "NextGraph will do this natively later." The test for any proposed
+addition: *does it compensate a real, exhibited NextGraph gap or bug?* If not, it
+does not belong here — build it in the consumer application, not in the polyfill.
+Corollary: a compensation whose gap is not actually exhibited on the target broker
+is dead weight, not defensive code — it should be removed, not kept "just in case."
+
 What the polyfill adds on top of the real SDK (each emulated for now, native as
 NextGraph matures):
 - Shared-wallet identity (one wallet for everyone; the current identity id is