fix(launcher): remove hard-coded Python 3.13 path

[s-qin]

fix(launcher): remove hard-coded Python 3.13 path

Problem

OpenSpan.bat hard-coded a fallback interpreter: C:\Python313\openspanw.exe (a renamed pythonw.exe copy). On machines with any other Python install (here: Python 3.14.3 with Tk 8.6), that file does not exist and the launcher dies with a raw missing-file path error — so after OpenSpan.exe is absent, OpenSpan cannot be started again.

Reproduction evidence:

old : start "" "C:\Python313\openspanw.exe" "%~dp0win\openspan.py"
     -> file not found
verified environment : Python 3.14.3, Tk 8.6 (pyw.exe Python Launcher)
after: OpenSpan.bat -> dynamically resolved pythonw.exe -> win\openspan.py -> GUI launched

Fix

OpenSpan.bat now resolves the interpreter dynamically — nothing version- or username-bound:

1. OpenSpan.exe next to the script is still preferred, always.
2. Otherwise the first usable GUI interpreter is picked, in this order:
   a. pyw.exe — Windows Python Launcher, console-less
   b. pythonw.exe on PATH
   c. python.exe on PATH (WindowsApps Store stub skipped)
   d. newest registered PythonCore install (pythonw preferred)
3. Every candidate is validated with import tkinter; a broken candidate is skipped, never trusted.
4. Clear errors via MessageBox: "no Python found" vs "Python found but tkinter missing" — never a raw missing-interpreter path error.
5. No renamed binaries, no requiring PATH edits, no hardcoded install dirs; quoting is safe for spaces.

Docs updated for the same change (minimal): BUILD.md (build command example is now version-agnostic: py -3 build_exe.py), build_exe.py docstring example, TECHNICAL_NOTES.md launch section.

Validation

Manually tested on real hardware (OpenSpan.exe absent):

• OpenSpan.bat starts the GUI successfully
• dynamically resolves the current pythonw.exe (Python 3.14.3 / Tk 8.6)
• no Python-3.13 dependency
• GUI appears and is functional

No CI pipeline configured in this repository (no .github/workflows); static checks passed (python -m py_compile, git diff --check).