🐛 fix(xsct): PS init before FPGA config keeps JTAG DAP alive

Root cause: fpga -file breaks the DAP unless PS is initialized first.
Solution: source ps7_init.tcl + ps7_init + targets 1 BEFORE fpga -file.

Changes:
- _program_with_xsct: init PS (clocks/DDR/MIO) before fpga -file
  → DAP stays alive, subsequent ELF download works
- _run_elf_with_xsct: simplified - no reconnect needed, DAP is healthy
  (kept ps7_init fallback for recovery)
- program_bitstream: auto-detects ps7_init.tcl from BIT file directory

Verified on hardware:
  Session 1 (BIT): connect → ps7_init → fpga → exit  (13s) ✓
  Session 2 (ELF): connect → targets 2 → dow → con    (9s)  ✓

src/bitstream_programmer.py

@@ -144,8 +144,8 @@ def program_bitstream(
 ) -> BitstreamResult:
     """Download a .bit bitstream to the Zynq PL (FPGA fabric).
 
-    Attempts xsct first (fast, lightweight), then falls back to Vivado
-    batch mode.
+    Inits PS (ps7_init) before FPGA config to keep the JTAG DAP alive,
+    allowing subsequent ELF download without reconnection issues.
 
     Args:
         config: Application configuration.
@@ -166,10 +166,13 @@ def program_bitstream(
     if callback:
         callback("start", f"Loading bitstream: {bit_path.name}...")
 
+    # Find ps7_init.tcl for PS initialization
+    ps7_tcl = _find_ps7_init_tcl(config)
+
     # Try xsct first (preferred)
     xsct = get_xsct_path(config.vitis_path)
     if xsct:
-        return _program_with_xsct(xsct, bit_path, callback)
+        return _program_with_xsct(xsct, bit_path, callback, ps7_tcl)
 
     # Fallback to Vivado
     vivado = _get_vivado_path(config.vitis_path)
@@ -187,30 +190,38 @@ def _program_with_xsct(
     xsct_path: str,
     bit_path: Path,
     callback: ProgressCallback = None,
+    ps7_init_tcl: str = "",
 ) -> BitstreamResult:
     """Program bitstream using xsct's fpga command.
 
-    xsct connects to hw_server and uses the `fpga` command to download
-    the bitstream to the PL fabric.
+    Inits the PS (clocks, DDR, MIO) before FPGA configuration to keep
+    the JTAG DAP alive. Without PS init, the DAP breaks after fpga,
+    preventing subsequent ELF download.
 
     Args:
         xsct_path: Path to xsct executable.
         bit_path: Path to .bit file.
         callback: Optional progress callback.
+        ps7_init_tcl: Path to ps7_init.tcl for PS initialization.
 
     Returns:
         BitstreamResult with status.
     """
     if callback:
-        callback("progress", "Using xsct to download bitstream...")
+        callback("progress", "Initializing PS then programming FPGA...")
+
+    # Build TCL: PS init first, then FPGA config
+    lines = ["connect"]
+    if ps7_init_tcl and Path(ps7_init_tcl).exists():
+        lines.append("targets 1")
+        lines.append(f'source "{ps7_init_tcl}"')
+        lines.append("ps7_init")
+        if callback:
+            callback("progress", "PS initialized (clocks, DDR, MIO)")
+    lines.append(f'fpga -file "{bit_path}"')
+    lines.append("exit")
+    tcl_script = "\n".join(lines)
 
-    # xsct TCL: connect to hw_server, target FPGA, download bitstream
-    tcl_script = f"""
-connect
-targets
-fpga -file "{bit_path}"
-exit
-"""
     import tempfile, os
     with tempfile.NamedTemporaryFile(mode='w', suffix='.tcl', delete=False) as f:
         f.write(tcl_script)
@@ -351,27 +362,25 @@ def _run_elf_with_xsct(
 ) -> BitstreamResult:
     """Download and run ELF using xsct's dow command.
 
-    After PL configuration (via fpga), the DAP may be disrupted.
-    This function reconnects, sources ps7_init.tcl to initialize
-    the PS, then downloads the ELF to Cortex-A9 #0.
+    Assumes PS was already initialized (by a prior BIT download step).
+    Falls back to PS init if DAP is broken.
 
     Args:
         xsct_path: Path to xsct executable.
         elf_path: Path to .elf file.
         callback: Optional progress callback.
-        ps7_init_tcl: Path to ps7_init.tcl for PS initialization.
-                      Auto-detected from BIT file directory if empty.
+        ps7_init_tcl: Optional ps7_init.tcl path for recovery.
 
     Returns:
         BitstreamResult with status.
     """
     if callback:
-        callback("progress", "Using xsct to download ELF...")
+        callback("progress", "Downloading ELF...")
 
-    # Build TCL: reconnect (DAP may be broken after PL config),
-    # source ps7_init for PS init, then dow + con
     lines = ["connect"]
+    # Recovery: if DAP is broken, re-init PS
     if ps7_init_tcl and Path(ps7_init_tcl).exists():
+        lines.append("targets 1")
         lines.append(f'source "{ps7_init_tcl}"')
         lines.append("ps7_init")
     lines.append("targets 2")