fix(build): compile C++ libraries carried in a STruC++ upload

[MatthewReed303]

Summary

An upload can carry ordinary C++ library folders under
core/generated/libraries/<name>/, but Makefile.strucpp could not build them:

• GEN_CPP := $(wildcard $(GENERATED_DIR)/*.cpp) globbed the top level only, so a
  library's sources were never compiled and the link failed on its symbols.
• Nothing put those libraries' headers on the include path, so a block's
  #include <Foo.h> did not resolve.
• The %.o pattern rule assumed a flat object tree, so a nested source had
  nowhere to build to.

Three changes:

• RESOURCE_LIB_INC derives an include root per library folder — its src/ when
  it has one, the folder itself otherwise — and adds them with addprefix -I.
• Sources are discovered with find, so any depth is covered. Wrapped in sort
  for a reproducible order across filesystems.
• The pattern rule gained mkdir -p $(@D), since
  libraries/foo/src/transport/bar.cpp now builds to
  $(BUILD_DIR)/libraries/foo/src/transport/bar.o.

An upload with no libraries/ directory compiles exactly as before — find
returns the same set the old wildcard did, and RESOURCE_LIB_INC is empty.

This is the runtime half of an editor change that lets a .stlib ship the C/C++
sources its blocks compile against; the Arduino half needs nothing from here.

Ticket

How it was tested

Built openplc-runtime in Docker from this branch, uploaded a program using a
library that ships its protocol sources as resources, and confirmed the runtime
compiled and loaded it:

-I core/generated/libraries/SensorKit/src -I core/generated/libraries/SensorNet/src
Compiling core/generated/libraries/SensorNet/src/transport/HostUdpTransport.cpp...
  -o build/libraries/SensorNet/src/transport/HostUdpTransport.o
Build complete: build/new_libplc.so
PLC started.

Every part of the change is exercised: the per-library include roots, recursive
discovery, and mkdir -p $(@D) for the nested objects. The loaded libplc_*.so
carries the library's symbols as real objects in the link.

Also confirmed an upload carrying no libraries/ directory builds unchanged.

Checklist

• bash scripts/run-pytest.sh passes
• pre-commit run clean
• Docs updated if behavior changed (README, CLAUDE.md, docs/)
• Follows docs/pr-reviews/PR_REVIEW_CHECKLIST.md