release: bump to v1.3.0 and document the signing step that was never performed

The tree stamped 1.2.0 in both configure.ac and src/clientversion.h, but
v1.2.0 is an annotated tag already pushed at fad05d3ab and dev is 27
commits past it. Both trees therefore produced CLIENT_VERSION 1020050 and
announced an identical /DragonX:1.2.0/ subversion, so a released binary
would have been indistinguishable from the tag on the wire, in
getnetworkinfo, and to the wallet's in-app updater -- destroying the only
provenance check users have: build the tag, compare the binary.

Bump MINOR rather than REVISION: the delta since v1.2.0 adds a subsystem
(RandomX stratum) and a new RPC (stratummine).

  configure.ac, src/clientversion.h  1.2.0 -> 1.3.0 (CLIENT_VERSION 1030050)
  doc/man/*.1                        version strings restamped
  contrib/debian/changelog           1.3.0 entry for the 27 commits
  src/chain.h                        stale "CLIENT_VERSION is 1010050" comment

Man page *content* still needs a real regeneration: util/gen-manpages.sh
requires help2man and built 1.3.0 binaries, so it belongs in the release
build, where it will also pick up the new stratum options.

doc/release-process.md is why v1.1.0 and v1.2.0 were tagged but never
became releases. Followed verbatim it produced a release the wallet
refuses to install:

  - it directed releases to a branch named `dragonx`, which does not
    exist; releases are cut on `master`
  - it never once mentioned signing, yet the updater pins an ed25519 key
    and sets kDaemonRequireSignature = true, so an unsigned release is
    refused outright and every user silently stays on their old daemon
  - it did not require the release tag to be annotated, and genbuild.sh
    calls `git describe` without --tags, so a lightweight tag stamps the
    build `v<older-tag>-<sha>` instead of the release version -- which is
    exactly what happened to v1.0.0 through v1.0.3
  - it referenced util/build-debian-package-ARM.sh, which is not in tree

Adds the signing and checksum-table steps, the annotated-tag requirement
with a `git describe` verification, and the rule that a new version must
exceed every existing tag including unpublished ones.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FU87LdsJZiZkfq1eXubpeo

doc/man/dragonx-cli.1

@@ -1,9 +1,9 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man 1.49.1.
-.TH DRAGONX-CLI "1" "August 2026" "dragonx-cli v1.2.0" "User Commands"
+.TH DRAGONX-CLI "1" "August 2026" "dragonx-cli v1.3.0" "User Commands"
 .SH NAME
-dragonx-cli \- manual page for dragonx-cli v1.2.0
+dragonx-cli \- manual page for dragonx-cli v1.3.0
 .SH DESCRIPTION
-DragonX RPC client version v1.2.0
+DragonX RPC client version v1.3.0
 .PP
 In order to ensure you are adequately protecting your privacy when using
 DragonX, please see <https://dragonx.is/security/>.