core: add the allowlist policy preset

The engine already honored requireAllowlist + allowlistMiss, and the
commerce example configured a seller allowlist by hand -- but there was no
reusable, named "only show allowlisted publishers" preset, the symmetric
twin of the denylist. Integrators (Tessera) punted on allowlist mode for
lack of one.

Add ALLOWLIST_POLICY (registry name "allowlist"): a publisher is shown only
when an authorized actor has contributed a user-allow for them, and every
unlisted publisher misses and is hidden across all four dimensions.
Administrative denials still apply on top, and protected actors (root,
moderators) are never gated -- the people who curate the list can always
see. Populate userAllow exactly as the denylist: signed contributions
resolved from relays (e.g. NIP-32 allow labels).

Exported from @bitgate/core, registered in POLICY_PRESETS, documented in the
README, and covered by presets.test.js (unlisted hidden, allowlisted shown,
protected bypasses).

Addresses the "allowlist / whitelist mode" item of the Tessera integration
feedback (docs/TODO.md).

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01BXQCvJPA6sVSQ5j8uBd9bB

README.md

@@ -81,9 +81,10 @@ await provider.ready;
 await provider.useSigner(window.nostr);   // BitLogin's NIP-07 provider
 ```
 
-Policy presets: `social`, `commerce`, `admin-only`. Every number in them is a
-starting point — see the [integration guide](docs/integration-guide.md) to
-write your own.
+Policy presets: `social`, `commerce`, `admin-only`, and `allowlist` (only
+allowlisted publishers are shown — the symmetric twin of a denylist, for
+invite-only or curated surfaces). Every number in them is a starting point —
+see the [integration guide](docs/integration-guide.md) to write your own.
 
 ## Quick start — JavaScript