Week 4-BN: guard the console snippets against bv being None

Every console snippet derives the repo path from bv.file.original_filename, so
with no database open bv is None and it fails with 'NoneType has no attribute
file'. Add 'assert bv is not None, "Open the .bndb first..."' to all 12 snippets
so the failure is explicit.

WEEK04/WEEK04-BN.md

@@ -143,6 +143,7 @@ cmake --build build
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 os.environ["PATH"] = "/opt/homebrew/bin:" + os.environ["PATH"]   # the console's PATH omits Homebrew
 proj = os.path.dirname(os.path.dirname(bv.file.original_filename))   # <repo>/<project>
 subprocess.run(["cmake", "-B", "build", "-G", "Ninja", "-DPICO_BOARD=pico2",
@@ -154,6 +155,7 @@ subprocess.run(["cmake", "--build", "build"], cwd=proj)
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 proj = os.path.dirname(os.path.dirname(bv.file.original_filename))   # <repo>/<project>
 subprocess.run(["cmake", "-B", "build", "-G", "Ninja", "-DPICO_BOARD=pico2",
                 "-DPICO_PLATFORM=rp2350", "-DCMAKE_BUILD_TYPE=Release"], cwd=proj)
@@ -164,6 +166,7 @@ subprocess.run(["cmake", "--build", "build"], cwd=proj)
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 proj = os.path.dirname(os.path.dirname(bv.file.original_filename))   # <repo>\<project>
 subprocess.run(["cmake", "-B", "build", "-G", "Ninja", "-DPICO_BOARD=pico2",
                 "-DPICO_PLATFORM=rp2350", "-DCMAKE_BUILD_TYPE=Release"], cwd=proj)
@@ -262,6 +265,7 @@ A `.bin` has no headers, so OpenOCD must be told the base address `0x10000000`.
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))   # <repo>
 bin_path = os.path.join(root, "0x0005_intro-to-variables", "build", "0x0005_intro-to-variables.bin")
 log = os.path.join(os.path.dirname(bin_path), "flash.log")
@@ -275,6 +279,7 @@ print("flashing in the background; log:", log)
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))   # <repo>
 bin_path = os.path.join(root, "0x0005_intro-to-variables", "build", "0x0005_intro-to-variables.bin")
 log = os.path.join(os.path.dirname(bin_path), "flash.log")
@@ -315,6 +320,7 @@ age: 43
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))   # <repo>
 bin_path = os.path.join(root, "0x0008_uninitialized-variables", "build", "0x0008_uninitialized-variables.bin")
 log = os.path.join(os.path.dirname(bin_path), "flash.log")
@@ -328,6 +334,7 @@ print("flashing in the background; log:", log)
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))   # <repo>
 bin_path = os.path.join(root, "0x0008_uninitialized-variables", "build", "0x0008_uninitialized-variables.bin")
 log = os.path.join(os.path.dirname(bin_path), "flash.log")
@@ -467,6 +474,7 @@ $env:BP_ADDR="0x10000234"; .\debug-server.ps1
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))   # <repo>
 subprocess.run(["pkill", "-TERM", "-f", "openocd"])          # stop any running server first
 log = os.path.join(root, "openocd.log")
@@ -480,6 +488,7 @@ print("OpenOCD started (pid", p.pid, "); log:", log)
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))   # <repo>
 subprocess.run(["taskkill", "/F", "/IM", "openocd.exe"])     # stop any running server first
 log = os.path.join(root, "openocd.log")
@@ -1040,6 +1049,7 @@ Part 4 left the Pico running the patched Project 1 image. Put the original Proje
 
    ```python
    import os, subprocess
+   assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
    root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))   # <repo>
    bin_path = os.path.join(root, "0x0008_uninitialized-variables", "build", "0x0008_uninitialized-variables.bin")
    log = os.path.join(os.path.dirname(bin_path), "flash.log")
@@ -1053,6 +1063,7 @@ Part 4 left the Pico running the patched Project 1 image. Put the original Proje
 
    ```python
    import os, subprocess
+   assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
    root = os.path.dirname(os.path.dirname(os.path.dirname(bv.file.original_filename)))   # <repo>
    bin_path = os.path.join(root, "0x0008_uninitialized-variables", "build", "0x0008_uninitialized-variables.bin")
    log = os.path.join(os.path.dirname(bin_path), "flash.log")
@@ -1389,6 +1400,7 @@ Hold **BOOTSEL**, plug in the Pico 2, drag `hacked.uf2` onto the **`RP2350`** dr
 
 ```python
 import os, subprocess
+assert bv is not None, "Open the .bndb in Binary Ninja first, or use the terminal command above."
 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")
 log = os.path.join(os.path.dirname(bv.file.original_filename), "flash.log")