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, 11 insertions, 0 deletions

diff --git a/README.md b/README.md
index 5b10332..bbb423e 100644
--- a/README.md
+++ b/README.md
@@ -200,6 +200,17 @@ identity.
 ; pattern that contains no date or time field at all is refused with a message,
 ; since it would print the same fixed text on every card.
 ; date_format = yyyy-MM-dd hh:mm
+; Optional. Extra subject prefixes that mark a message someone forwarded TO
+; you, drawn on the card as a received-forward mark. ADDED to the built-in
+; list rather than replacing it, so adding your own locale does not lose
+; Fwd:, Fw:, WG:, ENC:, RV: and TR:.
+; Bare words, without the colon. A prefix is matched only at the START of the
+; subject, case-insensitively, after any Re: chain, so a reply to a forward
+; still counts. Entries that are not a plain word are ignored.
+; This is display only: nothing is written to the message and nothing reaches
+; your mail server. It is unrelated to the "passed" tag, which is the Maildir
+; P flag and means "I forwarded this".
+; forward_prefixes = Doorst, VS, VL
 
 [completion]
 ; Optional. Extra content types offered after mimetype:, APPENDED to the