bible: embed only the Vulgate by default; other corpora opt-in

Split the bundled corpora so the default binary carries only the
public-domain Latin Vulgate. wuj/drb/grb move to corpora_optional/ and are
compiled in only with `-tags fullbible`, or dropped into the user corpora
dir at runtime. This keeps the distributed AGPL binary free of third-party
scripture text -- bible corpora are not covered by lectio's licence.

- bible: read corpus files/metadata from core + optional embed FS
  (embReadCorpus/embCorpusFiles); optionalFS is set only under the tag.
- config: default `versions` now reflects corpora actually available in the
  build (Vulgate first), so a vul-only binary offers no absent versions.
- Makefile: `make build` = vul only; `make build-full` embeds the rest;
  `make test` runs with -tags fullbible; check-corpora validates both dirs.
- tests: corpusmeta asserts on vul (the always-embedded corpus); the
  versions-default expectation follows availableVersions().

1 files changed, 3 insertions, 4 deletions

diff --git a/internal/bible/registry.go b/internal/bible/registry.go
index 28571ea..794b48f 100644
--- a/internal/bible/registry.go
+++ b/internal/bible/registry.go
@@ -35,10 +35,9 @@ func buildRegistry() {
 	}
 	regMeta = map[string]CorpusMeta{}
 	regUserTSV = map[string]string{}
-	// embedded first
-	ents, _ := corporaFS.ReadDir("corpora")
-	for _, e := range ents {
-		if code, ok := strings.CutSuffix(e.Name(), ".ini"); ok {
+	// embedded first (core FS = vul always; optional FS = rest under -tags fullbible)
+	for _, name := range embCorpusFiles() {
+		if code, ok := strings.CutSuffix(name, ".ini"); ok {
 			if m, ok := embeddedCorpusMeta(code); ok {
 				regMeta[code] = m
 			}