feat(modules): client extension slots (phase 3, slice 2)

The client twin of the server's declareSlot/registerExtension, and the same rule
in both halves: core declares a slot, only core declares one, and at most one
module fills it. Core renders <Slot name> and gets nothing back when the slot is
unfilled, so an instance with no module installed renders exactly what it
rendered before -- the same untouched-path guarantee withModuleNav makes.

A slot is named for a PLACE, never for a meaning. Core supplies the position and
the styling; the label, the target, the data and whether anything renders at all
are the module's. The moment core types a slot by its content it has re-acquired
the game semantics phase 3 exists to remove.

This is the one place the client registry is not fail-open. An unknown slot, a
non-component and a second fill all throw, matching checkExtensionShape
server-side, because a dropped nav row costs a link the viewer can reach another
way while a silently dropped extension is invisible to everyone including its
author. A throw is always a programming error and never a race: core declares in
its own bundle and every module chunk is a deferred script injected after it.

Reading stays fail-safe -- undeclared and unfilled both read null -- and a
filling component renders inside an error boundary. That asymmetry is where the
client differs from the server: a module route that throws costs the module's own
page, but an extension throws inside CORE's, and the whole reason core keeps
ownership of that page is that it stays usable.

Core decorates a slot through <Slot wrap>, not by asking whether it is filled.
The obvious alternative is right about the unfilled case and wrong about the
failed one -- the extension is filled, so the separator renders, and then the
component throws into the boundary and leaves the separator behind on its own.
wrap puts core's decoration inside the boundary where it shares the extension's
fate. Found in a browser, with the footer's separator, which is the only place
either could have been found.

MODULE_API_VERSION 1.1.0 -> 1.2.0, both halves: the two state ONE version.
Contract: docs/website/MODULE_API.md 3.7.

Co-Authored-By: Claude <noreply@anthropic.com>

client/src/modules/version.js

@@ -11,9 +11,13 @@
 // that the two files can drift, so a test asserts they agree
 // (client/test/moduleRegistry.test.js) rather than trusting a bump to remember
 // both.
+// 1.2.0 — `registry` gained `registerExtension` and core gained extension slots
+// (MODULE_API.md §3.7). The first change to window.__rg since 1.0.0, and an
+// addition: a module that never fills a slot is unaffected. The server half is
+// untouched and bumps anyway, for the reason below.
 // 1.1.0 — the server's ctx gained activity.log, users.getById, site.baseUrl and
 // the rate-limit factory (MODULE_API.md §2.3). Nothing on window.__rg changed,
 // but the two halves state ONE version: a module declares a single coreApi range
 // and is served one chunk, so a client that claimed 1.0.0 while the server
 // answered 1.1.0 would be two answers to one question.
-export const MODULE_API_VERSION = '1.1.0'
+export const MODULE_API_VERSION = '1.2.0'