Add dynit.h z/OS-compatible dynamic allocation interface

[mgrossmann]

Summary

Provide a z/OS-compatible dynit.h header with dynalloc() / dynfree() / dyninit() API for SVC 99 dynamic allocation.

This is the standard struct-based interface that z/OS programmers expect. Under the hood, the implementation uses crent370's existing SVC 99 infrastructure (TXT99, RB99, NewTXT99(), arrayadd(), __svc99()).

Motivation

• dynit.h is a well-known z/OS interface — offering it makes crent370 more familiar to mainframe developers
• crent370 already has all the building blocks (svc99.h, rb99.h, txt99.h, @@svc99.asm, 50+ text unit builders)
• The existing __dsalc() / __dsfree() are string-based; dynalloc() / dynfree() offer a struct-based alternative

Files

• include/dynit.h — public header with __dyn_t struct, constants, function declarations
• src/clib/dynalloc.c — dynalloc() implementation (one function per module for autocall)
• src/clib/dynfree.c — dynfree() implementation

Design Decisions

• dyninit() is a pure memset macro (no BREXX-style byte offset hacks)
• __blksize is unsigned short (not short) since BLKSIZE can exceed 32767
• __rbx and __emsgparmlist are void* for now (RBX99 support can be added later)
• Function names dynalloc/dynfree without __ prefix (z/OS standard, not crent internal)
• asm("DYNALLOC") / asm("DYNFREE") for correct external symbol names