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, 7 insertions, 0 deletions

diff --git a/src/marks.cpp b/src/marks.cpp
index 8777f84..d205ea3 100644
--- a/src/marks.cpp
+++ b/src/marks.cpp
@@ -57,6 +57,13 @@ QByteArray svg(Mark mark)
             "viewBox=\"0 0 16 16\"> <path fill=\"currentColor\" d=\"M 6.8,2.2 V 5.0 "
             "H 8.7 C 11.9,5.0 14.2,7.4 14.2,10.8 V 13.8 a 0.9,0.9 0 0 1 -1.75,0.28 C "
             "11.8,12.1 10.4,10.9 8.7,10.9 H 6.8 V 13.7 L 1.0,7.95 Z\"/> </svg>");
+    case Mark::ReceivedForward:
+        return QByteArray(
+            "<svg xmlns=\"http://www.w3.org/2000/svg\" width=\"16\" height=\"16\" "
+            "viewBox=\"0 0 16 16\"> <path fill=\"currentColor\" d=\"M 2.0,2.6 a "
+            "0.95,0.95 0 0 1 1.9,0 V 6.4 C 3.9,8.1 5.2,9.4 6.9,9.4 H 9.6 V 6.6 L "
+            "15.0,11.0 L 9.6,15.4 V 12.6 H 6.9 C 3.5,12.6 2.0,10.3 2.0,7.4 Z\"/> "
+            "</svg>");
     case Mark::ExpanderCollapsed:
         return QByteArray(
             "<svg xmlns=\"http://www.w3.org/2000/svg\" width=\"16\" height=\"16\" "