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.

issues/06-piping.md

@@ -1,25 +1,40 @@
 # 06 — Implement piping between commands
 
-**Status:** open (post-core)
-**Blocked by:** #03, #04
+**Status:** done
 
 ## Syntax
 
+Pipes are written inside backtick strings using `|`:
+
 ```lua
-local result = `ls -l` | `grep ".lua"`
+local r = `ls -l | grep ".lua" | wc -l`
 ```
 
+The `|` operator between backtick expressions remains Lua's bitwise OR — pipes only work *within* a single backtick command string.
+
 ## Implementation
 
-- Connect stdout of one child to stdin of the next via `pipe()` + `dup2()`
-- Build a pipeline of N processes, all forked and connected
-- Only capture stdout/stderr of the **final** process
-- `waitpid()` all children, return exit code of last process
+Implemented in `lcmd.c` with two new functions:
 
-## Challenge
+- **`split_pipeline()`** — scans the command string for `|` outside single/double quotes, splits into an array of stage strings (up to 64 stages). Reuses the same quote-tracking logic as `parse_argv()`.
 
-Lua uses `|` for bitwise OR. The parser needs to disambiguate:
-- `|` between two command expressions → pipe
-- `|` between numeric expressions → bitwise OR
+- **`exec_pipeline()`** — executes a multi-stage pipeline:
+  1. `parse_argv()` each stage
+  2. Creates N-1 inter-stage pipes + stdout/stderr capture pipes for the last stage
+  3. Forks N children with appropriate stdin/stdout wiring
+  4. Captures output from the last stage only
+  5. Returns `{code=last_exit_code, stdout=captured, stderr=captured}`
 
-This is resolvable because the parser knows the type of each subexpression — a backtick expression is always a command.
+`luaB_command()` calls `split_pipeline()` first. Single-stage commands (no `|`) fall through to the original single-command codepath unchanged.
+
+## Behaviour
+
+- Exit code is from the **last** pipeline stage (like bash)
+- Only the last stage's stdout/stderr are captured in the result table
+- Middle stages' stderr is inherited (goes to terminal)
+- Quoted `|` characters (single or double quotes) are not pipe separators
+- Empty pipeline stages (e.g. `cmd1 || cmd2` or leading/trailing `|`) are errors
+
+## Tests
+
+See `testes/lush/piping.lua`.