go-milter: build it patched not to keep every header

The security documentation's last open item of panel-dkim-signing (the
owner's decision, 2026-10-16): go-milter v0.4.1 added every header of a
session to a map of its own, emptied only on ABORT, and handed it to
Headers - which no milter of the panel reads. Postfix bounds one header,
not their number, so a message of nothing but headers up to the message
size limit was held whole by the library in the sender check, the signer
and the journal milter on 465/587 and in the inbound journal milter on port
25, where strangers send; the signer's 1 MiB refusal came after it was all
in.

third_party/go-milter is v0.4.1 from the module cache (BSD 2-Clause,
LICENSE kept) with session.go changed: CodeHeader hands the header to
Header and keeps nothing, CodeEOH hands Headers an empty map. PATCHES.md
says what and why, and how to take a new upstream version. go.mod replaces
the module with it; the Dockerfile copies third_party before "go mod
download". TestTheLibraryKeepsNoHeaders holds it through the library's own
client over two messages of one connection. Comments, security.md (the
accepted risk becomes a note), development.md's library table, the plan and
CHANGELOG follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

third_party/go-milter/PATCHES.md

@@ -0,0 +1,33 @@
+# go-milter v0.4.1, as SelfPost builds it
+
+This is github.com/emersion/go-milter at v0.4.1 (BSD 2-Clause, `LICENSE`),
+copied from the Go module cache, with one change. The main module takes it
+through a `replace` in its `go.mod`; the module path stays the upstream one.
+Left out of the copy: `cmd/` (a command-line tool SelfPost does not build) and
+the protocol notes (`milter-protocol*.txt`).
+
+## The change: headers are not kept
+
+`session.go`, `Process()`, cases `CodeHeader` and `CodeEOH`. Upstream adds
+every header of a session to a map (`m.headers`) that it empties only on
+ABORT, and hands that map to the backend's `Headers` at the end of the
+headers. Postfix bounds one header (60000 bytes with milters) but not their
+number, so a message of nothing but headers, up to `message_size_limit`,
+was held whole by the library in every milter that receives headers - the
+sender check, the DKIM signer and the journal milter on 465/587, the inbound
+journal milter on port 25 - on top of what the milters keep themselves
+(security review of panel-dkim-signing, 2026-10-16).
+
+Patched, `CodeHeader` hands each header to the backend's `Header` and keeps
+nothing; `CodeEOH` hands `Headers` an empty map. No SelfPost milter reads
+that map: the sender check and the signer keep the headers they need
+themselves (internal/milter, `senderSession`, `signSession`), and the journal
+milters do not implement `Headers`.
+
+The client side (`client.go`, used by the unit tests) is unchanged.
+
+## Taking a new upstream version
+
+Copy the files again, apply the change above, and keep this note. Read the
+new `session.go` first: if upstream bounds or drops the map itself, the
+patch - and this copy - may go.