feat(Settings): allow custom builds to extend generated settings pages

[DonLakeFlyer]

Description

Custom builds (QGC_CUSTOM_DIR) can now add, replace, reposition, or remove generated settings pages without overriding the stock generated QML.

How it works

• Page list overlay — <custom>/src/AppSettings/pages/SettingsPages.json is merged into the stock page list at configure time. Overlay entries support insertAfter/insertBefore, replace-in-place (matching name), and { "remove": "<name>" }. All malformed overlays (unknown keys, unknown anchors, duplicate qml outputs, missing names, missing page definitions) are hard configure errors.
• Page definitions — custom *.SettingsUI.json files shadow stock definitions of the same name.
• Custom settings groups — custom *.SettingsGroup.json fact metadata feeds the generator; a custom stem colliding with a stock SettingsManager accessor is rejected.
• Runtime registration — SettingsManager now derives from QQmlPropertyMap and exposes registerCustomSettingsGroup(accessor, group) (takes ownership; rejected groups are deleted). Called from the new QGCCorePlugin::registerCustomSettings hook so generated pages resolve QGroundControl.settingsManager.<accessor>.<fact>.
• CMake — when an overlay is active, the generated QML output list is computed via the generator's new --list-outputs mode instead of the hardcoded list.

Example

custom-example includes a complete working example: an overlay inserting a "Custom" page, its page definition, fact metadata, SettingsGroup subclass, and plugin registration.

Testing

• 19 new pytest cases covering overlay merge semantics, validation errors, custom metadata dirs, and CLI --list-outputs/generation
• New SettingsManagerTest C++ unit test covering registration, rejection paths, and ownership (rejected groups deleted)
• Stock and custom-example macOS builds verified

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.52%. Comparing base (f29efd3) to head (be37448).
⚠️ Report is 353 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Settings/SettingsManager.cc	47.61%	0 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15026      +/-   ##
==========================================
+ Coverage   25.47%   34.52%   +9.05%     
==========================================
  Files         769      945     +176     
  Lines       65912    85626   +19714     
  Branches    30495    39466    +8971     
==========================================
+ Hits        16788    29562   +12774     
+ Misses      37285    36421     -864     
- Partials    11839    19643    +7804     

Flag	Coverage Δ
unittests	34.52% <50.00%> (+9.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/API/QGCCorePlugin.h	77.27% <100.00%> (+22.72%)	⬆️
src/Settings/SettingsManager.cc	24.73% <47.61%> (+4.21%)	⬆️

... and 661 files with indirect coverage changes

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