ci: add build-prerelease workflow for CUDA RPMs on rpm.lair.cafe

Adds a manually-triggered workflow that builds CUDA-flavoured neuron
binaries and a CPU cortex binary, packages them as Fedora RPMs, signs
them, and rsyncs to the unstable channel at
https://rpm.lair.cafe/fedora/43/x86_64/unstable/. Mirrors the build
pipeline used by grenade/mistralrs-package.

Pipeline:
- prepare: derive {version,short_sha,commit_date} from the checkout;
  the prerelease Release stamp "0.1.YYYYMMDDgitSHORTSHA" sorts below
  the eventual "1" stable release.
- build-cortex: cargo build --release -p cortex-cli on a rust runner.
- build-neuron: matrix over ada (sm_89) and blackwell (sm_120) on
  cuda-13.0 runners; cargo build with features "cuda cudnn flash-attn"
  and CUDA_COMPUTE_CAP set per flavour.
- package-{cortex,neuron}: rpmbuild on the rpm runner against the new
  prebuilt-binary specs in rpm/.
- publish: import signing key, sign RPMs, rsync to oolon, createrepo_c
  --update, then regenerate packages.json for the UI.

New specs are prebuilt-binary variants — they consume the artifact
from the build job rather than running cargo at rpmbuild time. Each
helexa-neuron-{flavour} package Conflicts with the other flavours and
with helexa-neuron (the future source-build stable package) so one
flavour is installed at a time on a given host.

neuron crate gains cudnn and flash-attn feature flags forwarding to
the corresponding candle features, so the CI build command compiles
those kernels into the binary.

sccache is intentionally NOT used in the prerelease jobs — CUDA
compute cap isn't in its cache key, so flavours would mis-hit each
other. Each prerelease build is a clean cargo build.

Required Gitea secrets (already in place for cortex.spec / COPR
workflow):
- RPM_SIGNING_KEY, RPM_SIGNING_KEY_ID
- RSYNC_SSH_KEY

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -327,6 +327,18 @@ dependencies = [
  "zip",
 ]
 
+[[package]]
+name = "candle-flash-attn"
+version = "0.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "12512cf8e706744642e9a8579305a6ed1e44a0c636ce20c416cd5c519de19b7d"
+dependencies = [
+ "anyhow",
+ "candle-core",
+ "cudaforge",
+ "half",
+]
+
 [[package]]
 name = "candle-kernels"
 version = "0.10.2"
@@ -360,6 +372,7 @@ checksum = "f59d08c89e9f4af9c464e2f3a8e16199e7cc601e6f34538c2cfbb42b623b1783"
 dependencies = [
  "byteorder",
  "candle-core",
+ "candle-flash-attn",
  "candle-nn",
  "fancy-regex",
  "num-traits",