fix(comments): thread comment replies by /IRT reference, count comments like Acrobat

Replies were matched to their parent by /NM across the whole document.
/NM need not be unique: PyMuPDF names the first comment on every page
`fitz-A0`, so every reply in such a file was filed under page 1's
comment, and the other pages showed none. Both comment readers (render
inspect on open, annot summaries after an edit) now resolve /IRT to the
parent's page and /Annots position by object reference, and the
Comments panel, comment properties, the canvas thread, the engine's
comment summary and the comment_list tool thread by that.

The Comments header now counts comments without their replies, as
Acrobat does (28 rather than 44 for the reported file), and a reply's
thread line runs from the avatar above it (the comment's or the
previous reply's) to its own instead of a 12 px stub.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KvugvPrjZtPVudhh7cMa54

parity/acrobat-features.toml

@@ -3560,8 +3560,8 @@ milestone = "M3"
 status = "shipped"
 commands = ["comment.list"]
 tools = ["comment_list"]
-evidence = ["crates/ui-egui/tests/it/ui.rs::opening_a_pdf_shows_comments_and_bookmarks", "crates/ui-egui/tests/it/comments.rs::the_panel_posts_comments_and_replies", "crates/ui-egui/tests/it/comments.rs::page_groups_collapse_and_expand"]
-notes = "cards grouped by page with replies, status badges, selection, reply box and an Add a comment box; page groups fold and unfold from their chevron or with Collapse/Expand all pages in the panel's \"…\" menu (a comment selected on a folded page opens its group). Groups by author, type or colour don't fold yet"
+evidence = ["crates/ui-egui/tests/it/ui.rs::opening_a_pdf_shows_comments_and_bookmarks", "crates/ui-egui/tests/it/comments.rs::the_panel_posts_comments_and_replies", "crates/ui-egui/tests/it/comments.rs::page_groups_collapse_and_expand", "crates/ui-egui/tests/it/comments.rs::replies_follow_irt_references_when_names_repeat_on_every_page", "crates/engine/src/tests.rs::replies_thread_by_reference_when_names_repeat_across_pages"]
+notes = "cards grouped by page with replies (threaded by the `/IRT` reference, so files whose `/NM` repeats per page, as PyMuPDF writes them, thread correctly; the header counts comments without replies, as Acrobat does), status badges, selection, reply box and an Add a comment box; page groups fold and unfold from their chevron or with Collapse/Expand all pages in the panel's \"…\" menu (a comment selected on a folded page opens its group). Groups by author, type or colour don't fold yet"
 
 [[feature]]
 id = "comment.appearance-generation"