feat(core): add skipped fiber state for configuration gates

Fiber 新增 SKIPPED 态与 Skip/Skipped 方法：门禁为假的插件在 Apply 之前
即被排除并释放其作用域 Context，为互斥实现（cache 与 cache_memory 等）
同时挂载由内核择一激活铺路。

backend/core/fiber.go

@@ -27,6 +27,10 @@ const (
 
 	// FiberDisposed indicates the plugin has been completely unmounted and its context disposed.
 	FiberDisposed FiberState = "DISPOSED"
+
+	// FiberSkipped indicates the plugin never activated because its configuration gate
+	// evaluated to false, so an alternative provider took over.
+	FiberSkipped FiberState = "SKIPPED"
 )
 
 // Fiber wraps a Plugin instance with a dedicated scoped Context and manages its
@@ -134,6 +138,26 @@ func (f *Fiber) Load() error {
 	return nil
 }
 
+// Skip transitions a pending plugin to FiberSkipped and releases its scoped Context.
+// Active plugins are left untouched, which makes the call safe to replay on every
+// reconciliation pass, including for plugins mounted after the first gate evaluation.
+func (f *Fiber) Skip() error {
+	f.mu.Lock()
+	if f.state != FiberPending {
+		f.mu.Unlock()
+		return nil
+	}
+	f.state = FiberSkipped
+	f.mu.Unlock()
+
+	return f.ctx.Dispose()
+}
+
+// Skipped reports whether the plugin was excluded by its configuration gate.
+func (f *Fiber) Skipped() bool {
+	return f.State() == FiberSkipped
+}
+
 // Unload tears down the plugin: ACTIVE -> UNLOADING -> DISPOSED.
 func (f *Fiber) Unload() error {
 	f.mu.Lock()

backend/core/fiber_test.go

@@ -5,6 +5,7 @@ package core_test
 
 import (
 	"Wavelet/core"
+	"Wavelet/core/extpoints"
 	"context"
 	"reflect"
 	"testing"
@@ -108,3 +109,56 @@ func TestFiber_UnsatisfiedDependencyReturnsError(t *testing.T) {
 	assert.Error(t, err)
 	assert.Contains(t, err.Error(), "unsatisfied dependencies")
 }
+
+// gateConfig is the configuration section consumed by gatedPlugin below.
+type gateConfig struct {
+	Enabled bool `config:"enabled" env:"GATE_ENABLED"`
+}
+
+// gatedPlugin implements core.ConfigGatedPlugin: two instances with opposite
+// expectations model the mutually exclusive cache / cache_memory driver pair.
+type gatedPlugin struct {
+	name    string
+	enabled bool
+	applied bool
+}
+
+func (g *gatedPlugin) Name() string { return g.name }
+
+func (g *gatedPlugin) Apply(_ *core.Context) error {
+	g.applied = true
+	return nil
+}
+
+func (g *gatedPlugin) DeclareConfig() []extpoints.ConfigBinding {
+	return []extpoints.ConfigBinding{{Prefix: "gate", Target: &gateConfig{}}}
+}
+
+func (g *gatedPlugin) ConfigEnabled(view extpoints.ConfigView) bool {
+	return view.Bool("gate.enabled", false) == g.enabled
+}
+
+func TestFiberSkipMovesToSkippedStateAndDisposesScope(t *testing.T) {
+	root := core.NewContext(nil)
+	plugin := &gatedPlugin{name: "cache", enabled: true}
+	f := core.NewFiber(root, plugin)
+	require.Equal(t, core.FiberPending, f.State())
+
+	require.NoError(t, f.Skip())
+
+	assert.Equal(t, core.FiberSkipped, f.State())
+	assert.True(t, f.Skipped())
+	assert.False(t, plugin.applied, "a skipped plugin must never reach Apply")
+	assert.NoError(t, f.Unload(), "unloading a skipped fiber is a no-op")
+}
+
+func TestFiberSkipIsInertForActiveFibers(t *testing.T) {
+	root := core.NewContext(nil)
+	f := core.NewFiber(root, &gatedPlugin{name: "cache", enabled: true})
+	require.NoError(t, f.Load())
+
+	require.NoError(t, f.Skip())
+
+	assert.Equal(t, core.FiberActive, f.State(), "Skip only applies to pending fibers")
+	assert.False(t, f.Skipped())
+}