[F34 partial] mxaccess-asb: fix collect_asbidata_payloads + add Active flag

Investigation via examples/asb-relay.rs middleman captured the full
S→C bytes of a working PublishResponse from the .NET probe against
MxDataProvider. Decoder fix verified by regression test against the
captured fixture; one further wire-format gap surfaced and is filed.

Closed in this commit:

1. collect_asbidata_payloads filtered out empty <ASBIData/> elements
   so positional payload[N] indexing collapsed when Status was
   empty-but-present. The wire form for PublishResponse is:
     <Status><ASBIData/></Status>          ← empty placeholder
     <Values><ASBIData>{bytes}</ASBIData></Values>
   Our decoder lost the positional info and read Values as Status,
   then panicked on the malformed parse. Fix: always push every
   <ASBIData> element (empty or not) so payloads[0]=Status and
   payloads[1]=Values stay aligned. New regression test
   tests/publish_capture.rs runs the full decode chain over the
   captured wire bytes (305-byte frame at
   tests/fixtures/publish-response-with-value.bin) and asserts
   values.len() == 1.

2. MinimalMonitoredItem.active: Option<bool> + new with_active()
   constructor. The .NET reference's MxAsbDataClient.AddMonitoredItems
   defaults to active: true (cs:441). Without <Active>true</Active>
   on the wire, MxDataProvider treats the subscription as inactive
   and Publish polls return empty Values. Both binary build and
   canonical XML emitters now conditionally emit <Active> when
   active.is_some(). Shared push_monitored_item_body helper
   eliminates the duplicate MonitoredItem encoder between
   AddMonitoredItems and DeleteMonitoredItems builders.

3. SampleInterval unit: clarified as **milliseconds** in
   MinimalMonitoredItem.sample_interval doc + the example
   (sample_interval_ticks → sample_interval_ms = 1000). Matches the
   .NET reference's `ulong sampleInterval = 1000` default.

Open: F34's deeper finding — `MonitoredItem`'s wire schema is
DataContract field-suffix names (`activeField`, `bufferedField`,
`itemField`, `sampleIntervalField`, etc., per the per-session NBFX
dictionary the .NET probe declares), NOT XmlSerializer property
names (`Active`, `Buffered`, `Item`, `SampleInterval`). Our binary
NBFX builder still uses the property names, so MxDataProvider
silently fails to register monitored items — successField=true with
a 0-length Status array. The fix needs a complete rebuild of
build_add_monitored_items_request_body and
build_delete_monitored_items_request_body to use the field-suffix
names plus emit the *Specified siblings (activeFieldSpecified,
idFieldSpecified, etc.) as their own elements. The HMAC canonical
XML side is unaffected (XmlSerializer naming is correct there;
verified byte-equal to .NET via the F28 fixtures). Detailed in
design/followups.md F34's "Open" section.

Live verification of the F34-partial bonus context:
  - Read still returns 99 end-to-end via canonical XML signing.
  - AddMonitoredItems still returns Status[0] = 0 items
    (server doesn't recognize our DataContract-misnamed payload).
  - Publish still returns 0 values (the F34-open consequence).
  - All other 13 canonical-XML signed ops succeed at the request
    level (no SOAP faults, no HMAC rejections).

Workspace: mxaccess-asb 95 → 96 (+1 capture-driven decoder test);
default-feature clippy clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rust/crates/mxaccess-asb/src/xml_canonical.rs

@@ -507,6 +507,13 @@ fn emit_monitored_item(s: &mut String, item: &MinimalMonitoredItem) {
     // xmlns redeclaration since they're already in iom:2.
     emit_inline_item_identity(s, "    ", "Item", &item.item);
     emit_iom_text(s, "    ", "SampleInterval", &item.sample_interval.to_string());
+    // <Active> is *Specified-gated; emit only when the consumer
+    // opted in (None → not on the wire). MxDataProvider's
+    // Publish path requires Active=true to actually deliver values
+    // — F34, verified live 2026-05-06.
+    if let Some(active) = item.active {
+        emit_iom_text(s, "    ", "Active", if active { "true" } else { "false" });
+    }
     // ValueDeadband + UserData: default-Variant shape (type=0,
     // length=0, payload nil).
     emit_iom_default_variant(s, "    ", "ValueDeadband");