100%/C: Limits Monitoring (S2F45–F48, E30 §6.21)

New LimitMonitorStore keyed by VID; each entry is a vector of
LimitDefinition (LIMITID + upper/lower deadband as arbitrary Items).
S2F45/F46 set, S2F47/F48 read.  VLAACK validates each VID exists.

Four new SxFy in the catalog; codegen handles the nested
list-of-(VID, list-of-LimitDefinition) shape.  LimitDefinition is
defined in store/limits.hpp and referenced as external_struct so the
data model and the message codecs share one type.

The actual "value crossed limit" detection + CEID emission is left to
the application's set_value path (E30 §6.21 leaves *how* the equipment
detects crossings up to the implementer).

COMPLIANCE.md: Limits Monitoring Additional capability flips ✅.
Tests: 80 cases / 465 assertions.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

data/messages.yaml

@@ -415,6 +415,81 @@ messages:
                 - {name: name, shape: {kind: scalar, item_type: ASCII}}
                 - {name: code, shape: {kind: scalar, item_type: BINARY_BYTE}}
 
+  # S2F45 / S2F46 / S2F47 / S2F48 — Limits Monitoring (E30 §6.21).  Each
+  # VID may carry several LimitDefinitions (one per LIMITID), each with
+  # an upper / lower deadband expressed as an arbitrary Item.
+  - id: S2F45
+    stream: 2
+    function: 45
+    w: true
+    builder: s2f45_define_variable_limits
+    parser: parse_s2f45
+    body:
+      kind: list
+      struct_name: DefineLimitsRequest
+      fields:
+        - {name: dataid, shape: {kind: scalar, item_type: U4}}
+        - name: entries
+          shape:
+            kind: list_of
+            element:
+              kind: list
+              struct_name: VidLimitsEntry
+              fields:
+                - {name: vid, shape: {kind: scalar, item_type: U4}}
+                - name: limits
+                  shape:
+                    kind: list_of
+                    element:
+                      kind: list
+                      struct_name: LimitDefinition
+                      external_struct: true
+                      fields:
+                        - {name: limitid, shape: {kind: scalar, item_type: U1}}
+                        - {name: upper,   shape: {kind: scalar, item_type: ITEM}}
+                        - {name: lower,   shape: {kind: scalar, item_type: ITEM}}
+
+  - id: S2F46
+    stream: 2
+    function: 46
+    builder: s2f46_define_variable_limits_ack
+    body: {kind: scalar, item_type: BINARY_BYTE, enum: LimitMonitorAck, param: vlaack}
+
+  - id: S2F47
+    stream: 2
+    function: 47
+    w: true
+    builder: s2f47_variable_limit_attribute_request
+    parser: parse_s2f47
+    body:
+      kind: list_of
+      element: {kind: scalar, item_type: U4}
+      name: vids
+
+  - id: S2F48
+    stream: 2
+    function: 48
+    builder: s2f48_variable_limit_attribute_data
+    body:
+      kind: list_of
+      name: entries
+      element:
+        kind: list
+        struct_name: VidLimitsEntry
+        fields:
+          - {name: vid, shape: {kind: scalar, item_type: U4}}
+          - name: limits
+            shape:
+              kind: list_of
+              element:
+                kind: list
+                struct_name: LimitDefinition
+                external_struct: true
+                fields:
+                  - {name: limitid, shape: {kind: scalar, item_type: U1}}
+                  - {name: upper,   shape: {kind: scalar, item_type: ITEM}}
+                  - {name: lower,   shape: {kind: scalar, item_type: ITEM}}
+
   # S2F43 / S2F44 — Reset Spooling (E30 §6.22, configure spoolable streams).
   - id: S2F43
     stream: 2