chore/docs: move to GitHub as the single home; drop archived-spec references

Codeberg is being retired as the project's public site, so every reference now
points at GitHub. That includes the Go module path (codeberg.org/mix/selfpost →
github.com/mixeme/selfpost): leaving an import path on a host that is going
away would break `go get` and `go install`, so this is not only a docs change.
Touches go.mod, test/e2e/go.mod, all imports, Makefile MODULE, the -ldflags
version stamp in build/Dockerfile and docs/development.md, the licence headers
in the SVG/HTML assets, and README (no more primary/mirror pair).

Comments no longer cite the archived specification. "spec 7.6.1", "spec 5.1"
and friends pointed into docs/archive/specification-v1.0.md, which is marked as
not a source of truth; each is now a reference to the live document that owns
the subject — architecture.md (with section), product.md, security.md or the
README. The review only asked for the 7.x refs (code-review.md § 4), but 4/5/6/
8/9 had the same defect, so they went too. Comments only, no behaviour change.

Also closes the remaining review items: architecture.md gained a Code layers
section with the layer diagram (A2), and TestParseDelivery gained the exotic
mail.log cases (§ 3).

Fixes a bug that last test found: the delivery-line pattern matched status=
greedily, taking the *last* occurrence on the line. Postfix appends the remote
server's reply verbatim, so a rejection whose reply quoted "status=sent" was
filed as a delivered message in the send log. It now takes the first status=
after the recipient, which is the real field.

R7 (CONTRIBUTING.md) moved to roadmap 2.x — one developer, no external PR flow,
so the file would have no audience yet. R1 (compose image tag) and the git tag
stay in roadmap § v1.x as the release-commit steps.

gofmt/go vet clean on both modules; go test ./... green except the three known
Windows-only failures (file perms, backslash paths, renaming an open file).
Not exercised on the dev server — no Docker locally.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/assets/selfpost-stamp.svg

@@ -1,5 +1,5 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 330 150" width="330" height="150" role="img" aria-label="SelfPost — self-hosted SMTP relay">
-<!-- SelfPost · https://codeberg.org/mix/selfpost · AGPL-3.0
+<!-- SelfPost · https://github.com/mixeme/selfpost · AGPL-3.0
      Wordmark set in IBM Plex Sans (OFL) and converted to outlines: the
      file needs no font installed to render as drawn. -->
 <path d="M24.70 14.00 L27.97 14.00 A2.7 2.7 0 0 0 33.37 14.00 L36.63 14.00 A2.7 2.7 0 0 0 42.03 14.00 L45.30 14.00 A2.7 2.7 0 0 0 50.70 14.00 L53.97 14.00 A2.7 2.7 0 0 0 59.37 14.00 L62.63 14.00 A2.7 2.7 0 0 0 68.03 14.00 L71.30 14.00 A2.7 2.7 0 0 0 76.70 14.00 L79.97 14.00 A2.7 2.7 0 0 0 85.37 14.00 L88.63 14.00 A2.7 2.7 0 0 0 94.03 14.00 L97.30 14.00 A2.7 2.7 0 0 0 102.70 14.00 L105.97 14.00 A2.7 2.7 0 0 0 111.37 14.00 L114.63 14.00 A2.7 2.7 0 0 0 120.03 14.00 L123.30 14.00 A2.7 2.7 0 0 0 128.70 14.00 L131.97 14.00 A2.7 2.7 0 0 0 137.37 14.00 L140.63 14.00 A2.7 2.7 0 0 0 146.03 14.00 L149.30 14.00 A2.7 2.7 0 0 0 154.70 14.00 L157.97 14.00 A2.7 2.7 0 0 0 163.37 14.00 L166.63 14.00 A2.7 2.7 0 0 0 172.03 14.00 L175.30 14.00 A2.7 2.7 0 0 0 180.70 14.00 L183.97 14.00 A2.7 2.7 0 0 0 189.37 14.00 L192.63 14.00 A2.7 2.7 0 0 0 198.03 14.00 L201.30 14.00 A2.7 2.7 0 0 0 206.70 14.00 L209.97 14.00 A2.7 2.7 0 0 0 215.37 14.00 L218.63 14.00 A2.7 2.7 0 0 0 224.03 14.00 L227.30 14.00 A2.7 2.7 0 0 0 232.70 14.00 L235.97 14.00 A2.7 2.7 0 0 0 241.37 14.00 L244.63 14.00 A2.7 2.7 0 0 0 250.03 14.00 L253.30 14.00 A2.7 2.7 0 0 0 258.70 14.00 L261.97 14.00 A2.7 2.7 0 0 0 267.37 14.00 L270.63 14.00 A2.7 2.7 0 0 0 276.03 14.00 L279.30 14.00 A2.7 2.7 0 0 0 284.70 14.00 L287.97 14.00 A2.7 2.7 0 0 0 293.37 14.00 L296.63 14.00 A2.7 2.7 0 0 0 302.03 14.00 L305.30 14.00 A2.7 2.7 0 0 0 308.00 16.70 L308.00 20.01 A2.7 2.7 0 0 0 308.00 25.41 L308.00 28.73 A2.7 2.7 0 0 0 308.00 34.13 L308.00 37.44 A2.7 2.7 0 0 0 308.00 42.84 L308.00 46.16 A2.7 2.7 0 0 0 308.00 51.56 L308.00 54.87 A2.7 2.7 0 0 0 308.00 60.27 L308.00 63.59 A2.7 2.7 0 0 0 308.00 68.99 L308.00 72.30 A2.7 2.7 0 0 0 308.00 77.70 L308.00 81.01 A2.7 2.7 0 0 0 308.00 86.41 L308.00 89.73 A2.7 2.7 0 0 0 308.00 95.13 L308.00 98.44 A2.7 2.7 0 0 0 308.00 103.84 L308.00 107.16 A2.7 2.7 0 0 0 308.00 112.56 L308.00 115.87 A2.7 2.7 0 0 0 308.00 121.27 L308.00 124.59 A2.7 2.7 0 0 0 308.00 129.99 L308.00 133.30 A2.7 2.7 0 0 0 305.30 136.00 L302.03 136.00 A2.7 2.7 0 0 0 296.63 136.00 L293.37 136.00 A2.7 2.7 0 0 0 287.97 136.00 L284.70 136.00 A2.7 2.7 0 0 0 279.30 136.00 L276.03 136.00 A2.7 2.7 0 0 0 270.63 136.00 L267.37 136.00 A2.7 2.7 0 0 0 261.97 136.00 L258.70 136.00 A2.7 2.7 0 0 0 253.30 136.00 L250.03 136.00 A2.7 2.7 0 0 0 244.63 136.00 L241.37 136.00 A2.7 2.7 0 0 0 235.97 136.00 L232.70 136.00 A2.7 2.7 0 0 0 227.30 136.00 L224.03 136.00 A2.7 2.7 0 0 0 218.63 136.00 L215.37 136.00 A2.7 2.7 0 0 0 209.97 136.00 L206.70 136.00 A2.7 2.7 0 0 0 201.30 136.00 L198.03 136.00 A2.7 2.7 0 0 0 192.63 136.00 L189.37 136.00 A2.7 2.7 0 0 0 183.97 136.00 L180.70 136.00 A2.7 2.7 0 0 0 175.30 136.00 L172.03 136.00 A2.7 2.7 0 0 0 166.63 136.00 L163.37 136.00 A2.7 2.7 0 0 0 157.97 136.00 L154.70 136.00 A2.7 2.7 0 0 0 149.30 136.00 L146.03 136.00 A2.7 2.7 0 0 0 140.63 136.00 L137.37 136.00 A2.7 2.7 0 0 0 131.97 136.00 L128.70 136.00 A2.7 2.7 0 0 0 123.30 136.00 L120.03 136.00 A2.7 2.7 0 0 0 114.63 136.00 L111.37 136.00 A2.7 2.7 0 0 0 105.97 136.00 L102.70 136.00 A2.7 2.7 0 0 0 97.30 136.00 L94.03 136.00 A2.7 2.7 0 0 0 88.63 136.00 L85.37 136.00 A2.7 2.7 0 0 0 79.97 136.00 L76.70 136.00 A2.7 2.7 0 0 0 71.30 136.00 L68.03 136.00 A2.7 2.7 0 0 0 62.63 136.00 L59.37 136.00 A2.7 2.7 0 0 0 53.97 136.00 L50.70 136.00 A2.7 2.7 0 0 0 45.30 136.00 L42.03 136.00 A2.7 2.7 0 0 0 36.63 136.00 L33.37 136.00 A2.7 2.7 0 0 0 27.97 136.00 L24.70 136.00 A2.7 2.7 0 0 0 22.00 133.30 L22.00 129.99 A2.7 2.7 0 0 0 22.00 124.59 L22.00 121.27 A2.7 2.7 0 0 0 22.00 115.87 L22.00 112.56 A2.7 2.7 0 0 0 22.00 107.16 L22.00 103.84 A2.7 2.7 0 0 0 22.00 98.44 L22.00 95.13 A2.7 2.7 0 0 0 22.00 89.73 L22.00 86.41 A2.7 2.7 0 0 0 22.00 81.01 L22.00 77.70 A2.7 2.7 0 0 0 22.00 72.30 L22.00 68.99 A2.7 2.7 0 0 0 22.00 63.59 L22.00 60.27 A2.7 2.7 0 0 0 22.00 54.87 L22.00 51.56 A2.7 2.7 0 0 0 22.00 46.16 L22.00 42.84 A2.7 2.7 0 0 0 22.00 37.44 L22.00 34.13 A2.7 2.7 0 0 0 22.00 28.73 L22.00 25.41 A2.7 2.7 0 0 0 22.00 20.01 L22.00 16.70 A2.7 2.7 0 0 0 24.70 14.00 Z" fill="#F3EDE1" stroke="#12161C" stroke-width="0.8" stroke-linejoin="round" />