refactor: rename quartz-relay module to geode

The relay implementation now stands as its own module. Fitting brand
for a Nostr relay shipped alongside the Quartz library — a geode is a
rock that holds quartz inside.

- Rename module directory quartz-relay/ -> geode/.
- Rename Gradle module path :quartz-relay -> :geode (settings.gradle).
- Rename Kotlin package com.vitorpamplona.quartz.relay -> com.vitorpamplona.geode.
- Rename application name + main class binding to match.
- Update the relay's NIP-11 advertised name to "geode" and the
  software URL to /tree/main/geode. Test asserting the doc updated.
- Refresh comments / Main.kt usage line / config.example.toml header.
- Update consumers: quartz/build.gradle.kts dependency path, and
  quartz NostrClient tests that import the in-process RelayHub.

quartz/build.gradle.kts

@@ -181,11 +181,11 @@ kotlin {
                     implementation(libs.kotlin.test)
                     implementation(libs.kotlinx.coroutines.test)
 
-                    // In-process Nostr relay so JVM/Android host tests don't
-                    // need network access or a Rust toolchain. The
-                    // `relay.fixtures` package carries the test-only event
-                    // generators and corpus loader.
-                    implementation(project(":quartz-relay"))
+                    // In-process Nostr relay (geode) so JVM/Android host
+                    // tests don't need network access or a Rust toolchain.
+                    // The `geode.fixtures` package carries the test-only
+                    // event generators and corpus loader.
+                    implementation(project(":geode"))
                 }
             }