Give apk a version it can parse in CI, and say so when it cannot

ci.yml writes MOD_VER=ci into config.env, and MOD_VER is anvil-core's
PKG_VERSION. apk's version grammar wants a digit first, so mkpkg
refused it -- with "info field 'version' has invalid value", naming
neither the package nor the value nor where it came from. A free-form
version was fine for .ipk, which is why the line survived the
migration unread.

MOD_VER=0 now. It is pinned rather than left to the date default so
two runs of one tree produce the same bytes, and 0 says "not a
release" as plainly as the word did while ordering below every real
version.

emit_apk checks the version before calling mkpkg and names the
package, the offending value and the config.env setting to fix. The
grammar test covers the recipes, and could not have caught this: the
value comes from config.env, so anvil-core answers with whatever the
machine running the test happens to say -- a date, locally, always
valid. The new gate reads MOD_VER out of the workflow files
themselves, which is the only place that value is written down, with
a companion asserting a workflow still pins one at all so the check
cannot pass by having nothing to check.

busybox's pkg.conf expanded $BUSYBOX_BIN bare and let a failed
sha256sum through a pipeline. Sourced under `set -euo pipefail` it
therefore failed -- correctly skipping the recipe, since pkg_recipes
evaluates PKG_WHEN in a subshell, but printing "BUSYBOX_BIN: unbound
variable" twice into every CI log immediately above the first real
failure, where it reads like the cause. With that fixed every recipe
reads under the build's own flags, so the unsigned-recipe gate now
covers all of them rather than only the ones a build would make: a
recipe that cannot be read is dropped by that subshell rather than
reported, so the failure mode is a package silently leaving the feed.

Verified with the production key: 290 static, 135 replica, 47 signed
packages and a payload. MOD_VER=ci reproduced locally against the new
message, and MOD_VER=0 built and pruned clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013rbKEyLXrzSE6wm3iNApVp

.github/workflows/ci.yml

@@ -198,7 +198,13 @@ jobs:
         run: |
           cat > config.env <<'CFG'
           MOD_NAME=anvil
-          MOD_VER=ci
+          # 0, NOT "ci": MOD_VER is anvil-core's PKG_VERSION, and apk's version
+          # grammar wants a number first -- `ci` is rejected by mkpkg with
+          # "package version is invalid" and no clue which package or field.
+          # It is pinned rather than left to default so two runs of one tree
+          # produce the same bytes; 0 says "not a release" as plainly as the
+          # word did, and orders below every real one.
+          MOD_VER=0
           TARGET_MACHINE=Creator5Pro
           # No stock paths: bin/common.sh defaults them into vendor/, where
           # `make vendor` puts the pinned download. Setting them here would be