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, 12 insertions, 0 deletions

diff --git a/src/composewindow.h b/src/composewindow.h
index 2eaeeea..c8cc12a 100644
--- a/src/composewindow.h
+++ b/src/composewindow.h
@@ -189,6 +189,18 @@ signals:
     /// \p path is the file that was removed, absolute.
     void draftRemoved(const QString &path);
 
+    /// A send succeeded, and the message it answers should record that.
+    ///
+    /// Item 68. \p sourceMessageId is the Message-ID of the message replied to
+    /// or forwarded, \p tag is "replied" or "passed". The window emits rather
+    /// than writing, because a tag write belongs to the one applyTags path in
+    /// MainWindow and the composer owns no worker.
+    ///
+    /// Emitted only after the send itself succeeded: a failed send leaves the
+    /// source untouched, since the flag asserts that the mail went.
+    void sourceMessageAnswered(const QString &sourceMessageId,
+                               const QString &tag);
+
 protected:
     /// The one place the registry is told, whichever route closes the window.
     void closeEvent(QCloseEvent *event) override;