Make rich messages editable via InstantPage↔markdown round-trip

Reconstruct markdown source from a stored InstantPage to populate the edit
field, then re-classify on save (the inverse of the send path). Adds the
InstantPageToMarkdown converter, edit-field population and save-time
re-classification in ChatControllerLoadDisplayNode, and a shared InstantPage
previewText surfaced through MessageContentKind for reply/pinned/forward
previews. Documented in CLAUDE.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

submodules/TelegramStringFormatting/Sources/InstantPagePreviewText.swift

@@ -0,0 +1,177 @@
+import Foundation
+import TelegramCore
+
+extension RichText {
+    public func previewText() -> String {
+        switch self {
+        case .empty:
+            return ""
+        case let .plain(value):
+            return value
+        case let .bold(value):
+            return value.previewText()
+        case let .italic(value):
+            return value.previewText()
+        case let .underline(value):
+            return value.previewText()
+        case let .strikethrough(value):
+            return value.previewText()
+        case let .fixed(value):
+            return value.previewText()
+        case let .url(value, _, _):
+            return value.previewText()
+        case let .email(value, _):
+            return value.previewText()
+        case let .concat(values):
+            var result = ""
+            for value in values {
+                result.append(value.previewText())
+            }
+            return result
+        case let .`subscript`(value):
+            return value.previewText()
+        case let .superscript(value):
+            return value.previewText()
+        case let .marked(value):
+            return value.previewText()
+        case let .phone(value, _):
+            return value.previewText()
+        case .image:
+            //TODO:localize
+            return "Photo"
+        case let .anchor(value, _):
+            return value.previewText()
+        case .formula:
+            //TODO:localize
+            return "Fx"
+        case let .textCustomEmoji(_, alt):
+            return alt
+        case let .textAutoEmail(value), let .textAutoPhone(value), let .textAutoUrl(value), let .textBankCard(value), let .textBotCommand(value), let .textCashtag(value), let .textHashtag(value), let .textMention(value), let .textMentionName(value, _), let .textSpoiler(value):
+            return value.previewText()
+        }
+    }
+}
+
+extension InstantPageListItem {
+    public func previewText() -> String {
+        switch self {
+        case .unknown:
+            return ""
+        case let .text(text, num):
+            if let num, !num.isEmpty {
+                return "\(num). \(text.previewText())"
+            } else {
+                return text.previewText()
+            }
+        case let .blocks(blocks, num):
+            var blocksText = ""
+            for block in blocks {
+                if !blocksText.isEmpty {
+                    blocksText.append("\n")
+                }
+                blocksText.append(block.previewText())
+            }
+            if let num {
+                return "\(num). \(blocksText)"
+            } else {
+                return blocksText
+            }
+        }
+    }
+}
+
+extension InstantPageBlock {
+    public func previewText() -> String {
+        switch self {
+        case .unsupported:
+            return ""
+        case let .title(text):
+            return text.previewText()
+        case let .subtitle(text):
+            return text.previewText()
+        case let .authorDate(author, _):
+            return author.previewText()
+        case let .header(text):
+            return text.previewText()
+        case let .subheader(text):
+            return text.previewText()
+        case let .heading(text, _):
+            return text.previewText()
+        case .formula:
+            return "Fx"
+        case let .paragraph(text):
+            return text.previewText()
+        case let .preformatted(text, _):
+            return text.previewText()
+        case let .footer(text):
+            return text.previewText()
+        case .divider:
+            return "\n"
+        case .anchor:
+            return ""
+        case let .list(items, _):
+            var result = ""
+            for item in items {
+                if !result.isEmpty {
+                    result.append("\n")
+                }
+                result.append(item.previewText())
+            }
+            return result
+        case let .blockQuote(text, caption):
+            return text.previewText() + caption.previewText()
+        case let .pullQuote(text, caption):
+            return text.previewText() + caption.previewText()
+        case .image(_, _, _, _):
+            //TODO:localize
+            return "Photo"
+        case .video(_, _, _, _):
+            //TODO:localize
+            return "Video"
+        case .audio:
+            //TODO:localize
+            return "Audio"
+        case .cover:
+            return ""
+        case .webEmbed:
+            return ""
+        case .postEmbed:
+            return ""
+        case .collage:
+            return ""
+        case .slideshow:
+            return ""
+        case .channelBanner:
+            return ""
+        case .kicker:
+            return ""
+        case .table:
+            //TODO:localize
+            return "Table"
+        case .details:
+            return ""
+        case .relatedArticles:
+            return ""
+        case .map:
+            //TODO:localize
+            return "Map"
+        }
+    }
+}
+
+extension InstantPage {
+    public func previewText() -> String {
+        let maxLength: Int = 200
+        var result = ""
+        for block in self.blocks {
+            if !result.isEmpty {
+                result.append("\n")
+            }
+            result.append(block.previewText())
+            if result.count > maxLength {
+                break
+            }
+        }
+        return result
+    }
+}