Remove legacy STACKMAN_HAVE_CALL macro #16

Background

All 10 current platform implementations support stackman_call(), making the STACKMAN_HAVE_CALL conditional compilation macro unnecessary.

Current state

• Every platform header defines STACKMAN_HAVE_CALL 1
• Only used in tests/test.c to conditionally compile test_04/test_05
• Was defensive when not all platforms had stackman_call implemented

Proposal

Remove the macro and always compile the stackman_call tests:

1. Remove #ifndef STACKMAN_HAVE_CALL blocks from all platform headers
2. Remove #ifdef TEST_04 conditionals in tests/test.c
3. Update documentation if needed

Benefits

• Simplifies codebase
• Reduces conditional compilation complexity
• Makes API surface more consistent

Breaking changes

None - all platforms already have it, removing the macro just makes it unconditional.

Priority

Low - cleanup/refactoring task, no functional impact.

Identified during RISC-V implementation review.