Add developer example module

[yeraylois]

Summary

Adds a disabled-by-default ExampleModule as a heavily-commented reference implementation for developers learning how to create firmware modules.

This follows up on Discussion #10422:
#10422

What changed

• Adds src/modules/ExampleModule.h
• Adds src/modules/ExampleModule.cpp
• Registers the module in Modules.cpp
• Adds MESHTASTIC_EXCLUDE_EXAMPLE so the module is disabled by default
• Uses PRIVATE_APP portnum (no protobuf changes needed)

What does NOT change

• No generated protobuf files included
• No submodule bumps

Rationale

The current ReplyModule is useful but very small. It only demonstrates allocReply() and does not show the full lifecycle of a Meshtastic module.

ExampleModule is intentionally didactic and demonstrates:

• Lightweight construction
• setup() initialization
• SinglePortModule port filtering
• handleReceived() packet handling
• allocReply() response generation
• Avoiding multi-hop broadcast reply storms
• A simple module-local state variable

Runtime impact

None by default.

The module is excluded from normal builds with:

-DMESHTASTIC_EXCLUDE_EXAMPLE=1

Developers can enable it locally by removing or undefining that flag.

Testing

• Local prep completed for the example module and docs follow-up

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[CLAassistant]

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[yeraylois]

Force-pushed to clean up commit history.

What changed in the force push:

• Removed ~48 auto-generated .pb.* files that were accidentally included due to a nanopb version mismatch (0.4.9 vs 0.4.9.1)
• Removed the submodule bump to a local commit
• The commit now only contains the actual code changes:
  • ExampleModule.h / ExampleModule.cpp
  • Modules.cpp registration
  • configuration.h + platformio.ini exclusion flag
  • Minimal portnums.pb.h change with EXAMPLE_APP = 14

No functional changes, just a cleaner diff for review.

[caveman99]

Minimal portnums.pb.h change with EXAMPLE_APP = 14

Please do not include ANY generated files in PR's - these will generate merge conflicts of the nasty kind once the protobuf PR is merged. What about putting this example on PRIVATE_APP portnum?

[yeraylois]

Makes sense. I'll switch the example module to PRIVATE_APP, remove the generated portnums.pb.h change entirely so this PR contains no generated files, and close the protobuf PR since it won't be needed anymore.

[yeraylois]

Changes in this push:

• Switched ExampleModule from EXAMPLE_APP to PRIVATE_APP
• Removed the generated portnums.pb.h change entirely
• Closed the protobuf PR since it's no longer needed
• Updated PR description to remove the protobuf dependency

This PR now contain no generated files and avoid the merge-conflict risk you pointed out.

[yeraylois]

@caveman99 Updated this yesterday based on your feedback. Let me know if this looks like the right direction now.