Plan: Resolve Open TODOs

Current State

Multiple TODO/FIXME markers in project source code:

File	Comment	Action
`CubicSDRDefs.h`	`TODO: Make the waterfall resolutions an option.`	Convert to GitHub issue
`AppFrame.cpp`	`TODO: refactor these..`	Address during AppFrame split
`AppFrame.cpp`	`TODO: Move the stuff from there to here`	Address during AppFrame split
`AppFrame.cpp`	`TODO: Catch key-ups outside of original target`	Convert to GitHub issue
`DataTree.h`	`TODO: smarter way with templates?` (x2)	Convert to GitHub issue
`DataTree.cpp`	`TODO: forced cast in (char*) beware...`	Address during reinterpret_cast fix
`DataTree.cpp`	`TODO: stack recursion optimization` (x2)	Convert to GitHub issue
`DemodulatorThread.cpp`	`TODO: handle digital modems with audio output`	Convert to GitHub issue
`DemodulatorMgr.cpp`	`TODO: This is probably unnecessary and confusing`	Investigate and either fix or remove
`SoapySDRThread.cpp`	Various TODOs about timing and bandwidth (x3)	Convert to GitHub issues
`GainCanvas.cpp`	`TODO: if it not desirable, do not update in AGC mode`	Convert to GitHub issue
`ScopeCanvas.cpp`	`TODO: find out why frontbuffer drawing has stopped working in wx 3.1.0?`	Investigate; may be fixed in newer wxWidgets
`PrimaryGLContext.cpp`	`TODO: Better recording indicator...`	Convert to GitHub issue
`BookmarkView.cpp`	`TODO: keys for other actions?`	Convert to GitHub issue

For TODOs that will be addressed by other plans, resolve them during those tasks:
- AppFrame.cpp refactor TODOs → resolve during Split AppFrame.cpp
- DataTree.cpp forced cast TODO → resolve during Replace reinterpret_cast
For the remaining TODOs, create GitHub issues for each with:
- The original TODO text
- File/line reference
- Description of the feature request or bug
Remove the TODO comments from the code, replacing with issue references:
```
// See: https://github.com/cjcliffe/CubicSDR/issues/XXX
```
For DemodulatorMgr.cpp ("probably unnecessary and confusing"), investigate the code and either fix the issue or remove the dead code.

Zero TODO/FIXME comments remain in src/ (verified by grep -r "TODO\|FIXME" src/).
Each removed TODO either has a corresponding GitHub issue or was resolved by another plan.
Build succeeds and no functional regressions introduced.

File	Action
`src/CubicSDRDefs.h`	Remove TODO, add issue reference
`src/AppFrame.cpp`	Remove resolved TODOs during split
`src/DataTree.h`	Remove TODOs, add issue references
`src/DataTree.cpp`	Remove TODOs, add issue references
`src/DemodulatorThread.cpp`	Remove TODO, add issue reference
`src/DemodulatorMgr.cpp`	Investigate and resolve
`src/SoapySDRThread.cpp`	Remove TODOs, add issue references
`src/GainCanvas.cpp`	Remove TODO, add issue reference
`src/ScopeCanvas.cpp`	Remove TODO, add issue reference
`src/PrimaryGLContext.cpp`	Remove TODO, add issue reference
`src/BookmarkView.cpp`	Remove TODO, add issue reference