Add threaded comments support for XLSX format with parsing, serialization, and person management

- Added `threaded_comments` module with `ThreadedComment`, `ThreadedComments`, `Person`, `PersonList`, and `Mention` structures for modern Excel comment threads
- Implemented XML parsing for threaded comments and person lists with support for @mentions, replies, timestamps, and resolution status
- Implemented XML serialization for threaded comments and person lists with proper namespace handling

Author: DevExzh

docs/FEATURE_MATRIX.md

@@ -128,7 +128,7 @@ This document tracks the implementation status of features across all supported
 | Shapes/Drawing objects | ❌ | ❌ | ❌ | DrawingML shapes, text boxes, connectors |
 | External links | ❌ | ❌ | ❌ | Linked workbooks and external refs |
 | Data connections / Query tables | ❌ | ❌ | ❌ | External data connections |
-| Threaded comments | ❌ | ❌ | ❌ | Modern comment threads (`threadedComments`) |
+| Threaded comments | ✅ | ✅ | ✅ | Modern comment threads with @mentions, replies, and resolution status |
 | Pivot charts | ❌ | ❌ | ❌ | Charts bound to pivot caches |
 | Timeline controls | ❌ | ❌ | ❌ | Timeline slicers |
 | Workbook/worksheet views | ❌ | ❌ | ❌ | Custom views and sheet views |

src/ooxml/opc/constants.rs

@@ -98,6 +98,12 @@ pub mod content_type {
         "application/vnd.openxmlformats-officedocument.spreadsheetml.pivotCacheRecords+xml";
     pub const SML_PIVOT_TABLE: &str =
         "application/vnd.openxmlformats-officedocument.spreadsheetml.pivotTable+xml";
+    pub const SML_COMMENTS: &str =
+        "application/vnd.openxmlformats-officedocument.spreadsheetml.comments+xml";
+    pub const SML_THREADED_COMMENTS: &str = "application/vnd.ms-excel.threadedcomments+xml";
+    pub const SML_PERSONS: &str = "application/vnd.ms-excel.person+xml";
+    pub const SML_SHEET_METADATA: &str =
+        "application/vnd.openxmlformats-officedocument.spreadsheetml.sheetMetadata+xml";
 
     // Excel Binary File Format content types
     pub const XLSB_BIN: &str = "application/vnd.ms-excel.sheet.binary.macroEnabled.main";
@@ -220,6 +226,11 @@ pub mod relationship_type {
         "http://schemas.openxmlformats.org/officeDocument/2006/relationships/pivotCacheRecords";
     pub const PIVOT_TABLE: &str =
         "http://schemas.openxmlformats.org/officeDocument/2006/relationships/pivotTable";
+    pub const THREADED_COMMENTS: &str =
+        "http://schemas.microsoft.com/office/2017/10/relationships/threadedComment";
+    pub const PERSONS: &str = "http://schemas.microsoft.com/office/2017/10/relationships/person";
+    pub const SHEET_METADATA: &str =
+        "http://schemas.openxmlformats.org/officeDocument/2006/relationships/sheetMetadata";
 
     // Images and media
     pub const IMAGE: &str =

src/ooxml/xlsx/mod.rs

@@ -41,6 +41,7 @@ pub mod sparkline;
 pub mod styles;
 pub mod table;
 pub mod template;
+pub mod threaded_comments;
 pub mod views;
 pub mod workbook;
 pub mod worksheet;
@@ -84,3 +85,8 @@ pub use writer::{
     PageBreak as WriterPageBreak, PageSetup as WriterPageSetup, RichTextRun, SheetProtection,
     StylesBuilder, WorkbookProtection,
 };
+// Re-export threaded comments types
+pub use threaded_comments::{
+    Mention, Person, PersonList, ThreadedComment, ThreadedComments, read_persons,
+    read_threaded_comments, write_persons, write_threaded_comments,
+};

src/ooxml/xlsx/resources/metadata.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<metadata xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"
+  xmlns:xlrd="http://schemas.microsoft.com/office/spreadsheetml/2017/richdata"
+  xmlns:xda="http://schemas.microsoft.com/office/spreadsheetml/2017/dynamicarray">
+  <metadataTypes count="1">
+    <metadataType name="XLDAPR" minSupportedVersion="120000" copy="1" pasteAll="1" pasteValues="1"
+      merge="1" splitFirst="1" rowColShift="1" clearFormats="1" clearComments="1" assign="1"
+      coerce="1" cellMeta="1" />
+  </metadataTypes>
+  <futureMetadata name="XLDAPR" count="1">
+    <bk>
+      <extLst>
+        <ext uri="{bdbb8cdc-fa1e-496e-a857-3c3f30c029c3}">
+          <xda:dynamicArrayProperties fDynamic="1" fCollapsed="0" />
+        </ext>
+      </extLst>
+    </bk>
+  </futureMetadata>
+  <cellMetadata count="1">
+    <bk>
+      <rc t="1" v="0" />
+    </bk>
+  </cellMetadata>
+</metadata>

src/ooxml/xlsx/threaded_comments/mod.rs

@@ -0,0 +1,46 @@
+//! Threaded comments module for XLSX files.
+//!
+//! This module provides structures and functions for reading and writing
+//! threaded comments (modern Excel comment threads) in XLSX workbooks.
+//!
+//! Threaded comments are a modern feature introduced in Office 365 that
+//! support conversation threads, @mentions, and richer collaboration features.
+
+pub mod person;
+pub mod reader;
+pub mod writer;
+
+pub use person::{Mention, Person, PersonList};
+pub use reader::{read_persons, read_threaded_comments};
+pub use writer::{write_persons, write_threaded_comments};
+
+/// A threaded comment in an Excel worksheet.
+///
+/// Threaded comments support conversation-style threads with replies,
+/// mentions, timestamps, and resolution status.
+#[derive(Debug, Clone, Default)]
+pub struct ThreadedComment {
+    /// Cell reference (e.g., "A1")
+    pub cell_ref: Option<String>,
+    /// Unique identifier for this comment
+    pub id: String,
+    /// ID of the parent comment (for replies)
+    pub parent_id: Option<String>,
+    /// Person ID who authored this comment
+    pub person_id: String,
+    /// Comment text content
+    pub text: Option<String>,
+    /// Timestamp when comment was created/edited
+    pub date_time: Option<String>,
+    /// Whether this comment thread is marked as done/resolved
+    pub done: Option<bool>,
+    /// List of @mentions in the comment
+    pub mentions: Vec<Mention>,
+}
+
+/// Collection of threaded comments for a worksheet.
+#[derive(Debug, Clone, Default)]
+pub struct ThreadedComments {
+    /// List of threaded comments
+    pub comments: Vec<ThreadedComment>,
+}

src/ooxml/xlsx/threaded_comments/person.rs

@@ -0,0 +1,39 @@
+//! Person and mention data structures for threaded comments.
+
+/// A person who can author threaded comments.
+///
+/// Persons are tracked separately in a person list XML part and referenced
+/// by their unique ID in threaded comments.
+#[derive(Debug, Clone, Default)]
+pub struct Person {
+    /// Display name of the person
+    pub display_name: String,
+    /// Unique identifier (GUID)
+    pub id: String,
+    /// Optional user ID from identity provider
+    pub user_id: Option<String>,
+    /// Optional provider ID (e.g., Active Directory)
+    pub provider_id: Option<String>,
+}
+
+/// Collection of persons who can author comments in a workbook.
+#[derive(Debug, Clone, Default)]
+pub struct PersonList {
+    /// List of persons
+    pub persons: Vec<Person>,
+}
+
+/// An @mention within a threaded comment.
+///
+/// Mentions allow tagging specific people within comment text using @username syntax.
+#[derive(Debug, Clone, Default)]
+pub struct Mention {
+    /// Person ID being mentioned
+    pub mention_person_id: String,
+    /// Unique ID for this mention
+    pub mention_id: String,
+    /// Character offset where mention starts in text
+    pub start_index: u32,
+    /// Length of the mention text
+    pub length: u32,
+}