feat: flag what you answered, mark what was forwarded to you

Item 68, which turned out to be three things once its premise was
measured. The note asked to extend a "passed" subject rule to "Fw:";
there was no subject rule, and the correlation it rested on did not
exist. What did exist was a gap nobody had reported.

Reply and forward now flag their source. The Maildir R and P flags,
which every other client sets and notmuch reads back as "replied" and
"passed", had never been written here: measured on the developer's
index, all 317 "replied" and all 6 "passed" came from other clients.
ComposeWindow emits sourceMessageAnswered after a successful send and
MainWindow routes it through sendMessageTagChange, message-scoped and
off the undo stack, for the reason auto mark-read is: the flag records
that the mail went, and the send cannot be undone.

ComposeContext carries sourceMessageId rather than reusing inReplyTo,
which is deliberately empty on a forward so the recipient's client does
not file it under the thread it left. Keying on it made the "passed"
half dead code that compiled and never fired. A resumed draft is
excluded: its kind records how the file was opened, not what the user is
doing, so flagging on it would set R from a guess.

A received forward gets its own mark. Derived from the subject at paint
time, storing nothing and reaching no server, because "passed" means "I
forwarded this" and setting it from a guess would assert something false
on 222 existing messages. subjectIsForwarded() shares forwardSubject()'s
prefix table so the two cannot disagree, strips a Re: chain first, and
takes extra locale spellings from [general] forward_prefixes, which
extends the built-in table rather than replacing it.

A mutation survived the first round and corrected a claim in the code:
QRegularExpression::escape already makes a punctuation prefix inert, so
the word guard is not about pattern validity. It stops a configured "-"
matching "-: x". The comment and test say that now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LXCZFLXbAii5n5wtovpdhh

1 files changed, 5 insertions, 1 deletions

diff --git a/src/cardlayout.h b/src/cardlayout.h
index de4c41f..92edd96 100644
--- a/src/cardlayout.h
+++ b/src/cardlayout.h
@@ -68,6 +68,9 @@ struct CardLayout
         bool hasAttachment = false;
         bool passed = false;
         bool replied = false;
+        /// Someone forwarded this message TO the user (item 68). Derived from
+        /// the subject, so unlike the three above it corresponds to no tag.
+        bool receivedForward = false;
     };
 
     /// Width of the account accent bar down a thread card's left edge.
@@ -111,7 +114,7 @@ struct CardLayout
     QRect flagRect;
 
     /// The state marks after the subject, in this order: attachment, passed,
-    /// replied. Each is empty when its state does not apply.
+    /// replied, received-forward. Each is empty when its state does not apply.
     ///
     /// Separate rects rather than one strip, because each is independently
     /// present or absent and a strip would have to encode which. They are laid
@@ -120,6 +123,7 @@ struct CardLayout
     QRect attachmentRect;
     QRect passedRect;
     QRect repliedRect;
+    QRect receivedForwardRect;
 
     /// The side of a square mark on line two, derived from the card's font so
     /// the marks scale with the user's text size rather than being pinned to a