Implement piping between commands in backtick syntax (issue #06)

Add split_pipeline() to split command strings on unquoted | and
exec_pipeline() to fork N children connected by inter-stage pipes.
Only the last stage's stdout/stderr are captured; middle stages'
stderr is inherited. Exit code comes from the last stage.

testes/lush/piping.lua

@@ -0,0 +1,83 @@
+-- testes/lush/piping.lua
+-- Tests for piping between commands (issue #06).
+
+print "testing piping"
+
+-- basic pipe
+do
+  local r = `echo hello | cat`
+  assert(r.stdout == "hello\n", "basic pipe stdout: " .. r.stdout)
+  assert(r.code == 0)
+end
+
+-- multi-stage pipe
+do
+  local r = `printf "a\nb\nc\n" | grep b | cat`
+  assert(r.stdout == "b\n", "multi-stage pipe stdout: " .. r.stdout)
+end
+
+-- exit code is from last stage
+do
+  local r = `echo hello | sh -c "exit 42"`
+  assert(r.code == 42, "exit code from last stage: " .. r.code)
+end
+
+-- pipe with first stage failing: cat gets EOF, succeeds
+do
+  local r = `nonexistent_cmd_999 | cat`
+  assert(r.code == 0, "cat should succeed with EOF: " .. r.code)
+end
+
+-- quoted pipe character is NOT a pipe separator (double quotes)
+do
+  local r = `echo "a|b"`
+  assert(r.stdout == "a|b\n", "double-quoted pipe: " .. r.stdout)
+end
+
+-- single-quoted pipe character is NOT a pipe separator
+do
+  local r = `echo 'a|b'`
+  assert(r.stdout == "a|b\n", "single-quoted pipe: " .. r.stdout)
+end
+
+-- pipe with interpolation
+do
+  local ext = ".lua"
+  local r = `echo "test.lua test.py" | grep "${ext}"`
+  assert(r.stdout == "test.lua test.py\n", "pipe with interpolation: " .. r.stdout)
+end
+
+-- stderr from middle stages goes to terminal, not captured
+do
+  local r = `sh -c "echo err >&2; echo out" | cat`
+  assert(r.stdout == "out\n", "stdout through pipe: " .. r.stdout)
+  assert(r.stderr == "", "middle stderr not captured: '" .. r.stderr .. "'")
+end
+
+-- single command unchanged (regression check)
+do
+  local r = `echo hello`
+  assert(r.stdout == "hello\n")
+  assert(r.code == 0)
+end
+
+-- pipe preserves multi-line output
+do
+  local r = `printf "line1\nline2\nline3\n" | cat`
+  assert(r.stdout == "line1\nline2\nline3\n", "multiline pipe: " .. r.stdout)
+end
+
+-- pipe with whitespace around |
+do
+  local r = `echo hello   |   cat`
+  assert(r.stdout == "hello\n", "whitespace around pipe: " .. r.stdout)
+end
+
+-- last stage stderr is captured
+do
+  local r = `echo hello | sh -c "cat; echo piperr >&2"`
+  assert(r.stdout == "hello\n", "last stage stdout: " .. r.stdout)
+  assert(r.stderr == "piperr\n", "last stage stderr: " .. r.stderr)
+end
+
+print "OK"