refactor(rpn): split ConstantsProvider into Reader/Writer/Admin sub-interfaces (#wj)

Follows the same ISP-compliant pattern as VariableStore (VariableReader,
VariableWriter, VariablePersistence). The fat 7-method ConstantsProvider
is now split into:

- ConstantsReader: GetConstant, ListConstants, Count, HasConstant
- ConstantsWriter: SetConstant
- ConstantsAdmin: ClearConstants, ReloadBuiltInConstants
- ConstantsProvider: embeds all three for convenience

RPN.consts narrowed to ConstantsReader since RPN only reads constants;
GetConstants() asserts to ConstantsProvider for callers that need full access.
Added compile-time satisfaction checks for all sub-interfaces.

2 files changed, 29 insertions, 5 deletions

diff --git a/internal/rpn/constants.go b/internal/rpn/constants.go
index fd15c2a..d497076 100644
--- a/internal/rpn/constants.go
+++ b/internal/rpn/constants.go
@@ -11,17 +11,41 @@ import (
 	"sync"
 )
 
-// ConstantsProvider defines the interface for reading constant values.
-type ConstantsProvider interface {
+// ConstantsReader defines the interface for reading constant values.
+type ConstantsReader interface {
 	GetConstant(name string) (float64, bool)
 	ListConstants() []ConstantInfo
 	Count() int
 	HasConstant(name string) bool
+}
+
+// ConstantsWriter defines the interface for writing constant values.
+type ConstantsWriter interface {
 	SetConstant(name string, value float64) error
+}
+
+// ConstantsAdmin defines the interface for administrative constant operations.
+type ConstantsAdmin interface {
 	ClearConstants()
 	ReloadBuiltInConstants()
 }
 
+// ConstantsProvider combines ConstantsReader, ConstantsWriter, and ConstantsAdmin
+// for full constant storage access.
+type ConstantsProvider interface {
+	ConstantsReader
+	ConstantsWriter
+	ConstantsAdmin
+}
+
+// Ensure Constants implements all constant sub-interfaces at compile time.
+var (
+	_ ConstantsReader   = (*Constants)(nil)
+	_ ConstantsWriter   = (*Constants)(nil)
+	_ ConstantsAdmin    = (*Constants)(nil)
+	_ ConstantsProvider = (*Constants)(nil)
+)
+
 // ConstantInfo represents a single constant with its name and value.
 type ConstantInfo struct {
 	Name  string
diff --git a/internal/rpn/rpn_state.go b/internal/rpn/rpn_state.go
index e99a02d..abb9ab8 100644
--- a/internal/rpn/rpn_state.go
+++ b/internal/rpn/rpn_state.go
@@ -13,7 +13,7 @@ import (
 type RPN struct {
 	mu             sync.RWMutex
 	vars           VariableStore
-	consts         ConstantsProvider
+	consts         ConstantsReader
 	ops            OperationsProvider
 	opRegistry     *OperatorRegistry
 	assignHandler  *assignmentHandler
@@ -27,7 +27,7 @@ func NewRPN(vars VariableStore, reg ...*MetricRegistry) *RPN {
 	consts := NewConstants()
 	ops := NewOperations(vars, reg...)
 	ops.SetMode(FloatMode) // Set default mode
-	ops.SetConstants(consts) // Share the same ConstantsProvider
+	ops.SetConstants(consts) // Share the same constants provider
 	return &RPN{
 		vars:          vars,
 		consts:        consts,
@@ -42,7 +42,7 @@ func NewRPN(vars VariableStore, reg ...*MetricRegistry) *RPN {
 // GetConstants returns the constants provider.
 // This method is thread-safe for concurrent reads.
 func (r *RPN) GetConstants() ConstantsProvider {
-	return r.consts
+	return r.consts.(ConstantsProvider)
 }
 
 // GetMode returns the current calculation mode.