Hide wolfSPDM internals behind WOLFSPDM_LOCAL

[aidangarske]

Context

Internal helpers (transcript, KDF, message builders) are still exported because wolfTPM's src/spdm/unit_test.c links them through libwolftpm.

Origin

main...wolftpm-core

What is needed

• Mark internal functions WOLFSPDM_LOCAL (hidden visibility).
• Keep the few the unit tests need behind WOLFSPDM_TEST_API, exported only in test builds.
• Update the export allowlist.

Blockers and dependencies

Export allowlist from the size-guard issue.

Acceptance criteria

Standalone and wolfTPM builds export only the public API; wolfTPM make check still passes.