Implement prefix-based interactive commands via ! (issue #14)

Add !command syntax that runs commands with inherited terminal (no
capture). The lexer treats ! as a statement-starting token, reading
to end-of-line with the same interpolation/escape/pipe support as
backtick commands. The C function sets _ = {code=N, stdout="", stderr=""}.

testes/literals.lua

@@ -124,9 +124,12 @@ lexerror([['alo \98]], "<eof>")
 -- valid characters in variable names
 for i = 0, 255 do
   local s = string.char(i)
+  -- skip '!' which is now a valid statement starter (interactive commands)
+  if s == '!' then goto continue end
   assert(not string.find(s, "[a-zA-Z_]") == not load(s .. "=1", ""))
   assert(not string.find(s, "[a-zA-Z_0-9]") ==
          not load("a" .. s .. "1 = 1", ""))
+  ::continue::
 end
 
 

testes/lush/interactive.lua

@@ -0,0 +1,97 @@
+-- testes/lush/interactive.lua
+-- Tests for prefix-based interactive command execution (issue #14).
+
+print "testing interactive commands"
+
+-- ===== BASIC: _ IS SET CORRECTLY =====
+
+-- basic interactive command sets _
+!true
+assert(type(_) == "table")
+assert(_.code == 0)
+assert(_.stdout == "")
+assert(_.stderr == "")
+
+-- exit code preserved
+!false
+assert(_.code == 1)
+
+-- specific exit code
+!sh -c "exit 42"
+assert(_.code == 42)
+
+-- command not found
+!nonexistent_command_xyz_999
+assert(_.code == 127)
+
+-- ===== INTERPOLATION =====
+
+do
+  local name = "hello"
+!echo ${name}
+  assert(_.code == 0)
+  assert(_.stdout == "")
+end
+
+-- interpolation with expression
+do
+  local x = 21
+!sh -c "exit ${x * 2}"
+  assert(_.code == 42)
+end
+
+-- ===== PIPING =====
+
+-- exit code from last stage
+!echo hello | sh -c "exit 7"
+assert(_.code == 7)
+
+-- piping: first stage output goes to second
+!echo hello | cat
+assert(_.code == 0)
+
+-- ===== _ IS OVERWRITTEN =====
+
+!true
+assert(_.code == 0)
+!false
+assert(_.code == 1)
+
+-- ===== INSIDE LUA BLOCKS =====
+
+do
+!true
+  assert(_.code == 0)
+end
+
+do
+  if true then
+!true
+    assert(_.code == 0)
+  end
+end
+
+do
+  for i = 1, 3 do
+!true
+    assert(_.code == 0)
+  end
+end
+
+do
+  local function run_cmd()
+!true
+    return _.code
+  end
+  assert(run_cmd() == 0)
+end
+
+-- ===== EMPTY COMMAND (just whitespace after !) =====
+
+do
+  -- set _ to something first, then run empty interactive
+  !true
+  assert(_.code == 0)
+end
+
+print "OK"