Join continued lines with nothing, as shell does

A backslash-newline is removed entirely in shell; it does not become
whitespace. Joining with a space split tokens that bash keeps together, so
`"https\` + `://host/pkg.tar.gz"` — one URL to bash, confirmed by sourcing it —
arrived at the extractor as `https ://host/pkg.tar.gz` and matched nothing.
Another download skipped silently.

Contrived to write by hand, but the joiner was approximating shell semantics
rather than following them, and that gap is what the check keeps getting caught
by. Ordinary continuations already carry whitespace around the backslash, so
nothing else changes.

Verified: real definitions pass, the split-scheme URL is now caught, and
continuations with digests still pass in all three forms.

Author: jeremy

bin/ci

@@ -97,7 +97,12 @@ join_continuations() {
   local line acc=""
   while IFS= read -r line || [ -n "$line" ]; do
     if [ "${line%\\}" != "$line" ]; then
-      acc="${acc}${line%\\} "
+      # Join with nothing, matching shell: a backslash-newline is removed entirely, it
+      # does not become whitespace. Inserting a space would split tokens shell keeps
+      # together — `"https\` + `://host/x"` is one URL to bash but would arrive here as
+      # `https ://host/x` and match nothing, silently skipping that download. Ordinary
+      # continuations already carry their own whitespace around the backslash.
+      acc="${acc}${line%\\}"
     else
       printf '%s%s\n' "$acc" "$line"
       acc=""