fix: include C++ sources in program MD5

[manux81]

What changed

• Include the generated c_blocks.h and c_blocks_code.cpp sources in the program MD5 whenever the project contains C/C++ POUs.
• Use length-prefixed sections so the composite hash input is deterministic and unambiguous.
• Preserve the existing MD5(program.st) behavior for projects without C/C++ POUs.
• Reuse the same generated C-block artifacts for hashing and firmware/runtime bundle composition.
• Update the relevant API comments to describe the expanded hash scope.

Why this is necessary

C/C++ POU bodies are extracted into the originalCppPous sidecar before Structured Text transpilation. The transpiler replaces each C/C++ body with an ST bridge, while the actual user-authored code is generated separately as c_blocks_code.cpp.

The pipeline previously calculated the MD5 from program.st only. As a result, changing only the C/C++ implementation while leaving the POU interface unchanged produced the same program.st and therefore the same MD5. The compiled program could change without the runtime/debugger digest reflecting that change, defeating stale-program detection.

Hashing the actual generated C/C++ sidecars together with program.st ensures every compiled user-program source contributes to the digest.

Impact

• C/C++ implementation changes now produce a new PROGRAM_MD5.
• Runtime v4, simulator, and Arduino compilation paths use the same corrected digest.
• Projects containing no C/C++ POUs retain their previous digest and do not incur unnecessary cache invalidation.

Validation

• Added a regression test that compiles two projects with identical ST output and different C/C++ bodies, then verifies that their MD5 values differ.
• Added coverage confirming ST-only projects still hash the raw program.st content.
• npm test -- --runInBand --coverage=false src/backend/shared/compile/__tests__/pipeline.test.ts — 54 tests passed.
• ESLint passed for all changed TypeScript files.
• Prettier check passed for all changed files.
• git diff --check passed.

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