ci: pin march to x86-64-v2, drop upload-artifact to v3

Two CI fixes from run #276 (got all the way through tests + mingw cross-
compile + packaging, only failed on artifact upload):

march: workflow now sets CRAFTER_BUILD_MARCH=x86-64-v2 / MTUNE=generic;
project.cpp reads both and applies them to the lib + exe Configurations
so the self-rebuild and mingw cross-compile honor the same baseline.
v3 is unusable on the runner — Intel N5105 (Tremont) has no AVX2, so
a v3 bootstrap binary wouldn't even start. v2 (SSE4.2) runs on the SBC
and on every x86_64 CPU since ~2011.

upload-artifact: pinned to v3. v4+ uses a GHES-only API that Forgejo
Actions doesn't implement; the v3 action stays on the older API that
Forgejo supports.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/ci.yaml

@@ -10,6 +10,14 @@ on:
 jobs:
   build-test-release:
     runs-on: arch-latest
+    env:
+      # x86-64-v2 = SSE4.2 baseline; runs on the CI SBC (Intel N5105 / Tremont,
+      # no AVX) and on essentially every x86_64 CPU since ~2011. v3 is out —
+      # it requires AVX2 the SBC doesn't have, so the bootstrap binary
+      # wouldn't even run there. mtune=generic keeps codegen balanced across
+      # consumer CPUs.
+      CRAFTER_BUILD_MARCH: x86-64-v2
+      CRAFTER_BUILD_MTUNE: generic
     steps:
       - name: Install build dependencies
         run: |
@@ -83,7 +91,9 @@ jobs:
           ls -la dist/
 
       - name: Upload workflow artifacts
-        uses: actions/upload-artifact@v4
+        # v4+ uses a GHES-only upload API Forgejo Actions doesn't implement;
+        # v3 stays on the older API that works on Forgejo.
+        uses: actions/upload-artifact@v3
         with:
           name: crafter-build
           path: dist/