feat(kiosk): resolve constant plugin IDs, overrides and the CheckT helper

- kiosk.ReadIdentity: ID()/Requires() as literals or constants across packages and modules, read with go/parser, never run
- plugin minimums from go.mod requires; v0.0.0 is an error, a pseudo-version warns
- kiosk.Validate, kiosk.CheckT (no testing import)
- plugin:add reads IDs through kiosk.ReadIdentity, so const-ID plugins register

modules/kiosk/testdata/consts/README.md

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+# Consts
+
+A fixture plugin whose identity is spread over constants.

modules/kiosk/testdata/consts/go.mod

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+module example.test/sm-consts-plugin
+
+go 1.27.0
+
+require (
+	example.test/sm-other-plugin v0.2.0
+	git.golem15.com/golem15/summercms v0.5.3
+)
+
+replace git.golem15.com/golem15/summercms => ../framework
+
+replace example.test/sm-other-plugin => ../other

modules/kiosk/testdata/consts/ids/ids.go

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+// Package ids holds plugin ID constants for the consts fixture.
+package ids
+
+// Translate is a plugin ID no module of the fixture provides.
+const Translate = "acme.translate"

modules/kiosk/testdata/consts/nested/README.md

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+# Nested consts
+
+The nested fixture plugin of the consts module.

modules/kiosk/testdata/consts/nested/plugin.go

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+// Package nested is a test fixture: a second plugin in a subdirectory of
+// the consts module, requiring the root plugin. It is never built.
+package nested
+
+import root "example.test/sm-consts-plugin"
+
+// Plugin is the fixture plugin.
+type Plugin struct{}
+
+// ID returns the plugin ID.
+func (*Plugin) ID() string { return "acme.constsnested" }
+
+// Requires the root plugin of the same module.
+func (*Plugin) Requires() []string { return []string{root.PluginID} }

modules/kiosk/testdata/consts/plugin.go

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+// Package consts is a test fixture: ID() and Requires() return constants
+// of the same package, of another package in the module and of a package
+// in another module. kiosk reads it with go/parser; it is never built.
+package consts
+
+import (
+	"example.test/sm-consts-plugin/ids"
+	"example.test/sm-other-plugin"
+)
+
+// PluginID is the plugin ID, built from two constants.
+const PluginID = vendor + ".consts"
+
+// Plugin is the fixture plugin.
+type Plugin struct{}
+
+// ID returns PluginID.
+func (*Plugin) ID() string { return PluginID }
+
+// Requires a plugin whose module is not required (acme.translate) and the
+// plugin of the other module.
+func (*Plugin) Requires() []string {
+	return []string{ids.Translate, other.PluginID}
+}

modules/kiosk/testdata/consts/vendor.go

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+package consts
+
+const vendor = "acme"

modules/kiosk/testdata/framework/go.mod

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+// A stub of the framework module: fixture go.mod files replace the
+// framework with this directory, so go list runs offline.
+module git.golem15.com/golem15/summercms
+
+go 1.27.0

modules/kiosk/testdata/literal/plugin.yaml

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+# Generated by summer plugin:manifest. Do not edit; rerun the command.
+schema: 1
+id: acme.blog
+module: example.test/sm-acme-plugin
+package: example.test/sm-acme-plugin
+name: Blog
+description: Posts and categories for an acme application, with a feed.
+framework: v0.5.3

modules/kiosk/testdata/other/README.md

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+# Other
+
+A plugin in another module that the consts fixture requires.

modules/kiosk/testdata/other/go.mod

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+module example.test/sm-other-plugin
+
+go 1.27.0
+
+require git.golem15.com/golem15/summercms v0.5.3
+
+replace git.golem15.com/golem15/summercms => ../framework

modules/kiosk/testdata/other/plugin.go

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+// Package other is a test fixture: a plugin in its own module that the
+// consts fixture requires. It is never built.
+package other
+
+// PluginID is the plugin ID.
+const PluginID = "acme.other"
+
+// Plugin is the fixture plugin.
+type Plugin struct{}
+
+// ID returns PluginID.
+func (*Plugin) ID() string { return PluginID }
+
+// Requires returns no plugins.
+func (*Plugin) Requires() []string { return []string{} }