nx_buf_dyn_owned_candidate_t218.nx

nx_buf_dyn.nx -- dynamic growable byte buffer. Every formatter / builder / ELF emitter today picks a fixed upfront cap and bails on overflow. When the cap is wrong the caller has to recompile or re-mmap. This module gives them std::vector-style amortized growth: 2x doubling when full. API: buf = nx_bd_new(initial_cap) nx_bd_byte(buf, b) nx_bd_bytes(buf, src, n) nx_bd_u32(buf, v) -- LE nx_bd_u64(buf, v) -- LE nx_bd_str(buf, s) -- NUL-terminated, NUL not written nx_bd_strz(buf, s) -- NUL-terminated, NUL written nx_bd_clear(buf) -- len=0, keeps capacity buf.buf, buf.len -- direct access for emit Growth: 2x when full, never less than +64. Old bytes are memcpy'd to the new region; the old region is leaked (we don't have munmap discipline yet, and arenas reset wholesale anyway).