feat(compose): derive a reply's recipients and headers, item 123

ComposeContext, task 7 of the compose-and-send plan. Address parsing,
recipient derivation, the References chain, subject prefixing and account
resolution, as free functions over values so they test without a painter.

Recipient derivation was designed from the spec rather than transcribed: the
plan's draft omitted it and its tests could not compile, calling
QVERIFY(config.load(path)) against a void return.

Six defects found in review, each pinned by a test checked against the
mutation that breaks it:

- Message-ids reached GMime bare, and GMime writes an EMPTY header for a bare
  addr-spec rather than complaining. In-Reply-To and References both shipped
  blank, so every reply would have arrived as an orphan thread with nothing
  wrong to see locally. MessageBuilder now brackets on write, in the one place
  that composes those headers rather than in each caller.
- internet_address_to_string was called with FALSE for the encode flag, so a
  display name carrying a raw newline rendered with the newline intact. That
  is a header-injection primitive.
- A reply to the user's own message addressed the user. It now goes to that
  message's original recipients, mirroring their To/Cc split, which is what
  the Sent view and a follow-up on unanswered mail need.
- A From parsing to no mailbox left To empty, reachable from real mail
  ("From: Mailer Daemon"). MessageBuilder treats an empty recipient list as
  success, so the message would have been handed to the send command with
  nobody to deliver to and filed in Sent looking sent.
- The References header was split on whitespace alone, so a client's
  non-conformant "<a@x>,<b@y>" became one token and the bracket strip produced
  the fabricated id "a@x>,<b@y".
- Reply and forward prefixes were recognised in English only, doubling every
  AW:, SV:, WG: and Re[2]: a mixed-locale mailbox receives. Single-letter
  spellings are deliberately excluded: with R: recognised, "R: report on Q3"
  reads as a prefix and a genuine first reply threads nowhere.

The mailbox-only guard in parseAddressHeader survived its first mutation
check, because removing it still yields no recipients: the invalid GObject
cast makes GMime's own assertion return NULL. That is undefined behaviour
papered over by an assertion G_DISABLE_CHECKS compiles out, so the test now
asserts on the emitted critical rather than on the count. Registering the log
handler on a NULL domain catches nothing; the criticals carry "GLib-GObject"
and "gmime".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LoaLBowZ6w1JNx6SEhDP1L

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