Week 4-BN: flash from the BN console without blocking it (Popen, not run)

subprocess.run blocks the console until OpenOCD exits (and can hang it if the
probe is contended). Use subprocess.Popen with output to flash.log and
start_new_session, then poll p.poll() or read the log. Verified: Popen returns in
~1 ms, flash completes ~2 s later with 'Verified OK'. Applied to Steps 21 and 29.

WEEK04/WEEK04-BN.md

@@ -840,25 +840,33 @@ age: 70
 
 **43 became 70, permanently, with one byte changed and no source code.**
 
-> **Faster: flash over the Debug Probe (no BOOTSEL).** The repo's `flash.sh` writes the raw `.bin` straight into XIP flash over SWD (`program <bin> 0x10000000 verify reset exit`), so you never touch BOOTSEL or a UF2. Run it from the Binary Ninja console:
+> **Faster: flash over the Debug Probe (no BOOTSEL).** The repo's `flash.sh` writes the raw `.bin` straight into XIP flash over SWD (`program <bin> 0x10000000 verify reset exit`), so you never touch BOOTSEL or a UF2. Run it from a terminal (`./flash.sh <bin>`), or from the Binary Ninja console **without freezing it** — use `subprocess.Popen`, which returns immediately, and send OpenOCD's output to a log file. (`subprocess.run` blocks the console until the flash finishes; do not use it here.)
 >
 > ```python
 > import os, subprocess
 > root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))
 > bin_path = os.path.join(os.path.dirname(bv.file.original_filename), "0x0005_intro-to-variables-h.bin")
-> subprocess.run([os.path.join(root, "flash.sh"), bin_path])
+> log = os.path.join(os.path.dirname(bv.file.original_filename), "flash.log")
+> p = subprocess.Popen([os.path.join(root, "flash.sh"), bin_path],
+>                      stdout=open(log, "w"), stderr=subprocess.STDOUT, start_new_session=True)
+> print("flashing in the background; log:", log)
 > ```
 >
-> Or the OpenOCD call directly, if you would rather not depend on the script:
+> The console is free the moment this returns. Check it with `print(p.poll())` (`None` = still running, `0` = done) or read `flash.log` — success ends with `** Verified OK **`. (Verified: `Popen` returns in ~1 ms; the flash itself takes ~2 s.)
+>
+> The same non-blocking form without the script:
 >
 > ```python
 > import os, subprocess
 > ocd = os.path.expanduser("~/.pico-sdk/openocd/0.12.0+dev")
 > bin_path = os.path.join(os.path.dirname(bv.file.original_filename), "0x0005_intro-to-variables-h.bin")
-> subprocess.run([f"{ocd}/openocd", "-s", f"{ocd}/scripts",
+> log = os.path.join(os.path.dirname(bv.file.original_filename), "flash.log")
+> p = subprocess.Popen([f"{ocd}/openocd", "-s", f"{ocd}/scripts",
 >     "-f", "interface/cmsis-dap.cfg", "-f", "target/rp2350.cfg",
 >     "-c", "adapter speed 5000",
->     "-c", f"program {bin_path} 0x10000000 verify reset exit"])
+>     "-c", f"program {bin_path} 0x10000000 verify reset exit"],
+>     stdout=open(log, "w"), stderr=subprocess.STDOUT, start_new_session=True)
+> print("flashing in the background; log:", log)
 > ```
 >
 > **The Debug Probe is single-owner.** If Binary Ninja is still attached (the `debug-server.sh` OpenOCD is running), the flash cannot grab the probe. Detach in Binary Ninja and stop that OpenOCD first:
@@ -1226,13 +1234,16 @@ python ..\uf2conv.py 0x0008_uninitialized-variables-h.bin ^
 
 Or run the conversion from the Binary Ninja console, exactly as in Step 20 (`os.chdir` to the build dir, then `runpy.run_path("../../uf2conv.py", run_name="__main__")` with `sys.argv` set to the arguments above).
 
-Hold **BOOTSEL**, plug in the Pico 2, drag `hacked.uf2` onto the **`RP2350`** drive. Or flash the `.bin` over the Debug Probe with SWD — no BOOTSEL — from the console, exactly as in Step 21 (stop any running OpenOCD first):
+Hold **BOOTSEL**, plug in the Pico 2, drag `hacked.uf2` onto the **`RP2350`** drive. Or flash the `.bin` over the Debug Probe with SWD — no BOOTSEL — from the console, exactly as in Step 21 (stop any running OpenOCD first, and use `Popen`, not `run`, so the console is not blocked):
 
 ```python
 import os, subprocess
 root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))
 bin_path = os.path.join(os.path.dirname(bv.file.original_filename), "0x0008_uninitialized-variables-h.bin")
-subprocess.run([os.path.join(root, "flash.sh"), bin_path])
+log = os.path.join(os.path.dirname(bv.file.original_filename), "flash.log")
+p = subprocess.Popen([os.path.join(root, "flash.sh"), bin_path],
+                     stdout=open(log, "w"), stderr=subprocess.STDOUT, start_new_session=True)
+print("flashing in the background; log:", log)
 ```
 
 ### Step 30: Verify