fix(spec): drive fish mock with head -n 1 instead of read+/dev/stdin

fish 3.x's `read` builtin closes stdin after the first line when run
from a `-c` script with a piped stdin (fish-shell/fish-shell#5714).
The previous `read -l LINE < /dev/stdin` worked around it on Linux but
on macOS BSD `/dev/stdin` opens via /dev/fd/0, which EOFs on the second
iteration and silently killed the loop — every fish case after the
first failed with a broken pipe (and an empty exception message).

Replace the `read` loop with `head -n 1` per iteration. The simulator
is strictly synchronous (one line in, one response out), so `head`'s
read-ahead never eats data the next iteration would need.

Author: claude

src/tabular/shell/fish.ecr

@@ -1,7 +1,16 @@
 <% completers.each do |comp| -%>
 <%= comp %> completion <%= bin %> | source
 <% end %>
-while read -l LINE < /dev/stdin
+# fish 3.x's `read` builtin closes stdin after the first line when invoked
+# from a `-c` script with a piped stdin. The historical workaround was
+# `read -l LINE < /dev/stdin`, but on macOS BSD that opens a fresh fd via
+# /dev/fd/0 which EOFs on the second iteration and silently kills the loop.
+# `head -n 1` sidesteps the buffering quirk on both platforms; relying on
+# the simulator being synchronous (one line in / one response out) keeps
+# its read-ahead from eating subsequent lines.
+while true
+	set -l LINE (head -n 1)
+	test -z "$LINE"; and break
 	complete -C "<%= prog %> $LINE"
 	printf '\0'
 end