feat(ci): Add Fedora RPM package build workflow

Build and publish RPM packages for Fedora using rpkg and official
rust-packaging macros. GPG sign packages with Ed25519 repository key
and deploy to Forgejo package registry.

Publishes packages to organised groups:
- continuwuity (binary): base group (stable/dev/branch-name)
- continuwuity-debuginfo: GROUP-debug
- continuwuity (source RPM): GROUP-src

Workflow triggers on pushes to relevant paths and version tags (v*).
Tagged releases use clean version numbers (v1.2.3 becomes 1.2.3-1)
while branch builds use git SHA versioning.

Include GPG public key for package verification and documentation
for RPM repository configuration and installation methods.

fedora/RPM-GPG-KEY-continuwuity.asc

@@ -0,0 +1,16 @@
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+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+=dxDr
+-----END PGP PUBLIC KEY BLOCK-----