refactor(clink): clink_offer metadata via ClinkOfferTag + DSL builder

Brings the kind-0 clink_offer field in line with the sibling fields' structure
instead of a raw string constant written to content only:
- New ClinkOfferTag (TAG_NAME/assemble/parse) under nip01Core/metadata/tags.
- clinkOffer() TagArrayBuilder DSL extension in TagArrayBuilderExt.
- MetadataEvent uses ClinkOfferTag.TAG_NAME and dual-writes it as a kind-0 tag
  in updateOrDeleteTagNames (NIP-1770 pattern), like lud16/nip05; drops the
  ad-hoc CLINK_OFFER_PROPERTY constant.

UpdateMetadataTest now also asserts the tag is emitted. quartz tests pass.

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/metadata/MetadataEvent.kt

@@ -28,6 +28,7 @@ import com.vitorpamplona.quartz.nip01Core.core.TagArrayBuilder
 import com.vitorpamplona.quartz.nip01Core.core.builder
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.AboutTag
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.BannerTag
+import com.vitorpamplona.quartz.nip01Core.metadata.tags.ClinkOfferTag
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.DisplayNameTag
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.Lud06Tag
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.Lud16Tag
@@ -88,9 +89,6 @@ class MetadataEvent(
         const val KIND = 0
         const val FIXED_D_TAG = ""
 
-        // CLINK Offers discovery key in kind-0 content (mirrors the NIP-05 `clink_offer` key).
-        const val CLINK_OFFER_PROPERTY = "clink_offer"
-
         fun createAddress(pubKey: HexKey): Address = Address(KIND, pubKey, FIXED_D_TAG)
 
         fun createAddressATag(pubKey: HexKey): ATag = ATag(KIND, pubKey, FIXED_D_TAG, null)
@@ -252,7 +250,7 @@ class MetadataEvent(
             nip05?.let { addIfNotBlank(currentMetadata, Nip05Tag.TAG_NAME, it) }
             lnAddress?.let { addIfNotBlank(currentMetadata, Lud16Tag.TAG_NAME, it) }
             lnURL?.let { addIfNotBlank(currentMetadata, Lud06Tag.TAG_NAME, it) }
-            clinkOffer?.let { addIfNotBlank(currentMetadata, CLINK_OFFER_PROPERTY, it) }
+            clinkOffer?.let { addIfNotBlank(currentMetadata, ClinkOfferTag.TAG_NAME, it) }
         }
 
         // For https://github.com/nostr-protocol/nips/pull/1770
@@ -267,6 +265,7 @@ class MetadataEvent(
             currentMetadata[Nip05Tag.TAG_NAME]?.let { nip05(it.text) } ?: run { remove(Nip05Tag.TAG_NAME) }
             currentMetadata[Lud16Tag.TAG_NAME]?.let { lud16(it.text) } ?: run { remove(Lud16Tag.TAG_NAME) }
             currentMetadata[Lud06Tag.TAG_NAME]?.let { lud06(it.text) } ?: run { remove(Lud06Tag.TAG_NAME) }
+            currentMetadata[ClinkOfferTag.TAG_NAME]?.let { clinkOffer(it.text) } ?: run { remove(ClinkOfferTag.TAG_NAME) }
         }
 
         private fun addIfNotBlank(

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/metadata/TagArrayBuilderExt.kt

@@ -23,6 +23,7 @@ package com.vitorpamplona.quartz.nip01Core.metadata
 import com.vitorpamplona.quartz.nip01Core.core.TagArrayBuilder
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.AboutTag
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.BannerTag
+import com.vitorpamplona.quartz.nip01Core.metadata.tags.ClinkOfferTag
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.DisplayNameTag
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.Lud06Tag
 import com.vitorpamplona.quartz.nip01Core.metadata.tags.Lud16Tag
@@ -51,3 +52,5 @@ fun TagArrayBuilder<MetadataEvent>.lud06(lud06: String) = addUnique(Lud06Tag.ass
 fun TagArrayBuilder<MetadataEvent>.banner(banner: String) = addUnique(BannerTag.assemble(banner))
 
 fun TagArrayBuilder<MetadataEvent>.pronouns(pronouns: String) = addUnique(PronounsTag.assemble(pronouns))
+
+fun TagArrayBuilder<MetadataEvent>.clinkOffer(offer: String) = addUnique(ClinkOfferTag.assemble(offer))

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip01Core/metadata/tags/ClinkOfferTag.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip01Core.metadata.tags
+
+import com.vitorpamplona.quartz.utils.ensure
+
+/** CLINK Offers pointer (`noffer1…`) advertised in a kind-0 profile, mirroring the NIP-05 `clink_offer` key. */
+class ClinkOfferTag {
+    companion object {
+        const val TAG_NAME = "clink_offer"
+
+        fun parse(tag: Array<String>): String? {
+            ensure(tag.size > 1) { return null }
+            ensure(tag[0] == TAG_NAME) { return null }
+            ensure(tag[1].isNotEmpty()) { return null }
+            return tag[1]
+        }
+
+        fun assemble(offer: String) = arrayOf(TAG_NAME, offer)
+    }
+}

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip01Core/metadata/UpdateMetadataTest.kt

@@ -205,7 +205,10 @@ class UpdateMetadataTest {
         // written into kind-0 content under the spec's `clink_offer` key
         assertEquals(true, event.content.contains("\"clink_offer\":\"$noffer\""))
 
-        // and parses back out
+        // and dual-written as a kind-0 tag (NIP-1770 pattern), like the other fields
+        assertContentEquals(arrayOf("clink_offer", noffer), event.tags.firstOrNull { it.firstOrNull() == "clink_offer" })
+
+        // and parses back out of the content
         val metadata = event.contactMetaData()
         assertNotNull(metadata)
         assertEquals(noffer, metadata.clinkOffer)