feat(client): add pvget_fields for field-filtered GET requests

Support the equivalent of `pvget -r "field(value,alarm)"` by encoding a
pvRequest with the requested field names. When fields is empty, sends
the default all-fields request.

Adds PvaClient::pvget_fields() and the lower-level pvget_fields() function.

Author: physwkim

spvirit-client/src/client.rs

@@ -6,6 +6,7 @@ use crate::auth::{resolved_authnz_host, resolved_authnz_user};
 use crate::search::resolve_pv_server;
 use crate::transport::{read_packet, read_until};
 use crate::types::{PvGetError, PvGetOptions, PvGetResult};
+use spvirit_codec::spvd_encode::encode_pv_request;
 use spvirit_codec::epics_decode::{
     PvaPacket, PvaPacketCommand, decode_op_response_status as codec_decode_op_response_status,
 };
@@ -134,7 +135,19 @@ pub async fn establish_channel(
     })
 }
 
+/// Convenience wrapper: GET with no field filtering.
 pub async fn pvget(opts: &PvGetOptions) -> Result<PvGetResult, PvGetError> {
+    pvget_fields(opts, &[]).await
+}
+
+/// GET with optional field filtering.
+///
+/// If `fields` is empty, requests all fields (equivalent to `-r ""`).
+/// Otherwise, encodes a pvRequest like `field(value,alarm,timeStamp)`.
+pub async fn pvget_fields(
+    opts: &PvGetOptions,
+    fields: &[&str],
+) -> Result<PvGetResult, PvGetError> {
     let target = resolve_pv_server(opts).await?;
 
     let conn = establish_channel(target, opts).await?;
@@ -147,12 +160,17 @@ pub async fn pvget(opts: &PvGetOptions) -> Result<PvGetResult, PvGetError> {
     } = conn;
 
     let ioid = 1u32;
-    // Match EPICS pvget GET init payload (extra pvRequest bytes observed in capture).
+    let pv_request = if fields.is_empty() {
+        // Empty pvRequest — request all fields
+        vec![0xfd, 0x02, 0x00, 0x80, 0x00, 0x00]
+    } else {
+        encode_pv_request(fields, is_be)
+    };
     let get_init_req = encode_get_request(
         sid,
         ioid,
         0x08,
-        &[0xfd, 0x02, 0x00, 0x80, 0x00, 0x00],
+        &pv_request,
         version,
         is_be,
     );

spvirit-client/src/lib.rs

@@ -52,7 +52,7 @@ pub use pva_client::{
 pub use pvlist::PvListSource;
 
 // --- Re-exports: client core ---
-pub use client::{ChannelConn, build_client_validation, establish_channel, pvget};
+pub use client::{ChannelConn, build_client_validation, establish_channel, pvget, pvget_fields};
 pub use format::{OutputFormat, RenderOptions, format_output};
 pub use search::{SearchTarget, build_auto_broadcast_targets, resolve_pv_server, search_pv};
 pub use transport::read_packet;

spvirit-client/src/pva_client.rs

@@ -233,6 +233,16 @@ impl PvaClient {
         low_level_pvget(&opts).await
     }
 
+    /// Fetch a PV with field filtering (equivalent to `pvget -r "field(value,alarm)"`).
+    pub async fn pvget_fields(
+        &self,
+        pv_name: &str,
+        fields: &[&str],
+    ) -> Result<PvGetResult, PvGetError> {
+        let opts = self.opts(pv_name);
+        crate::client::pvget_fields(&opts, fields).await
+    }
+
     // ─── pvput ───────────────────────────────────────────────────────────
 
     /// Write a value to a PV.