Add the connector contract, reusable nodes and per-port intervals

Connectors are the device-facing node class third parties write, so the
surface they build against is versioned and documented: ConnectorNode carries
a declared contract version, a polling loop that publishes only what changed
and reports health around it, and parameters whose credential fields are
marked x-secret so the editor offers the secrets store instead of a text box.
They are found through the fluksio.node_types entry point group, with the
package's own metadata as the manifest. docs/connectors/ has the contract and
the authoring guide; connector-skeleton/ is a working one to copy.

The controller no longer knows what any node type is: start, stop and
report_health are protocol methods on Node, and the built-ins were migrated to
them first, so the hooks a connector implements are the ones the engine has
been driving all along.

Marking a node reusable moves its source to _lib/ and points the node at it by
name. Other flows instantiate it with their own ports and settings, one fix
reaches all of them, and a shared source still in use cannot be deleted.

Ports gained an interval: an output publishes, and an input wakes its node, at
most every n seconds. State keeps the latest value, so only the delivery is
skipped, and pressing Run is never throttled.

Also fixes autosave sending no version on its first save of a session, which
made every flow saved more than once conflict with itself.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/client/schemas.gen.ts

@@ -343,6 +343,26 @@ export const HistoryPointSchema = {
     description: 'One numeric value a message carried, and when.'
 } as const;
 
+export const LibraryNodeSchema = {
+    properties: {
+        name: {
+            type: 'string',
+            title: 'Name'
+        },
+        used_by: {
+            items: {
+                type: 'string'
+            },
+            type: 'array',
+            title: 'Used By'
+        }
+    },
+    type: 'object',
+    required: ['name'],
+    title: 'LibraryNode',
+    description: 'A node source shared across flows, and who is using it.'
+} as const;
+
 export const MessageSchema = {
     properties: {
         message: {
@@ -398,6 +418,12 @@ export const MessageSpecSchema = {
         dtype: {
             '$ref': '#/components/schemas/DType',
             default: 'float'
+        },
+        interval: {
+            type: 'number',
+            minimum: 0,
+            title: 'Interval',
+            default: 0
         }
     },
     type: 'object',
@@ -408,7 +434,11 @@ export const MessageSpecSchema = {
     the flow name at load time; empty means the port is unbound.
 :param port: The identifier the node function sees. Defaults to the last
     segment of \`\`name\`\`, so unqualified flows read naturally.
-:param dtype: Payload type, validated on every message that passes through.`
+:param dtype: Payload type, validated on every message that passes through.
+:param interval: Deliver at most every this many seconds; 0 is every time.
+    On an output it holds back publishing, on an input it holds back waking
+    the node. The value is never lost — state keeps the latest — only the
+    delivery is skipped.`
 } as const;
 
 export const MessageValueSchema = {
@@ -492,6 +522,17 @@ export const NodeDef_InputSchema = {
             },
             type: 'array',
             title: 'Provides'
+        },
+        source_ref: {
+            anyOf: [
+                {
+                    type: 'string'
+                },
+                {
+                    type: 'null'
+                }
+            ],
+            title: 'Source Ref'
         }
     },
     type: 'object',
@@ -541,6 +582,17 @@ export const NodeDef_OutputSchema = {
             },
             type: 'array',
             title: 'Provides'
+        },
+        source_ref: {
+            anyOf: [
+                {
+                    type: 'string'
+                },
+                {
+                    type: 'null'
+                }
+            ],
+            title: 'Source Ref'
         }
     },
     type: 'object',
@@ -583,12 +635,29 @@ export const NodeStatusPublicSchema = {
                 }
             ],
             title: 'Error'
+        },
+        health: {
+            type: 'string',
+            enum: ['ok', 'degraded', 'down'],
+            title: 'Health',
+            default: 'ok'
+        },
+        health_detail: {
+            anyOf: [
+                {
+                    type: 'string'
+                },
+                {
+                    type: 'null'
+                }
+            ],
+            title: 'Health Detail'
         }
     },
     type: 'object',
     required: ['id'],
     title: 'NodeStatusPublic',
-    description: 'Whether a node loaded, and why not.'
+    description: 'Whether a node loaded, and how its connection is doing.'
 } as const;
 
 export const NodeTypeInfoSchema = {
@@ -614,6 +683,17 @@ export const NodeTypeInfoSchema = {
             type: 'boolean',
             title: 'Has Source',
             default: false
+        },
+        plugin: {
+            anyOf: [
+                {
+                    type: 'string'
+                },
+                {
+                    type: 'null'
+                }
+            ],
+            title: 'Plugin'
         }
     },
     type: 'object',
@@ -733,6 +813,18 @@ export const SecretValueSchema = {
     title: 'SecretValue'
 } as const;
 
+export const ShareRequestSchema = {
+    properties: {
+        lib_name: {
+            type: 'string',
+            title: 'Lib Name'
+        }
+    },
+    type: 'object',
+    required: ['lib_name'],
+    title: 'ShareRequest'
+} as const;
+
 export const TokenSchema = {
     properties: {
         access_token: {