feat(agera): derive mounted state from the graph instead of counting subscribers

- replace the subscriber counters with a presence model: mounted state is never stored, it is a question asked of the graph - does this node have a live path up to an effect - memoized per drain and invalidated by any edge edit
- deliver mount and unmount as one level change from a coalescing queue at quiescent boundaries: churn inside a turn is silent, sources mount before their dependents and unmount after them
- move the layer behind two lazy sockets wired on first `mountable()` use, so a signal-only bundle drops it entirely: `signal` -17%, `nanoviews` -4%
- `onMounted` and the new `isMounted` live in `signal.ts` over a plain listener list; `node.subsCount` and `node.mounted` are gone
- an `onMounted` listener registered while the signal is already mounted now gets `true` at the next boundary instead of waiting for the next cycle
- fix late contagion and late `mountable()` never mounting a signal, a cold computed unmounting a signal under a live subscriber, stuck `mounted(true)` on discarded subscriptions, listeners skipped or served twice when one un-subscribes or registers during a fire, and delivery lost for other signals when a listener throws

Author: dangreen

packages/agera/.size-limit.json

@@ -3,19 +3,19 @@
     "name": "All publics",
     "path": "dist/index.js",
     "import": "*",
-    "limit": "2.63 kB"
+    "limit": "2.72 kB"
   },
   {
     "name": "Signal",
     "path": "dist/index.js",
     "import": "{ signal }",
-    "limit": "1.61 kB"
+    "limit": "1.34 kB"
   },
   {
     "name": "Minimal set",
     "path": "dist/index.js",
     "import": "{ signal, computed, effect }",
-    "limit": "1.8 kB"
+    "limit": "1.51 kB"
   },
   {
     "name": "Popular set",

packages/agera/oxlint.config.ts

@@ -17,6 +17,7 @@ export default defineConfig({
   rules: {
     'typescript/no-invalid-void-type': 'off',
     'eslint/no-return-assign': 'off',
-    'eslint/no-multi-assign': 'off'
+    'eslint/no-multi-assign': 'off',
+    'eslint/no-param-reassign': 'off'
   }
 })

packages/agera/src/effect.spec.ts

@@ -19,7 +19,8 @@ import {
   boundDeferScope,
   startScope,
   stopScope,
-  observe
+  observe,
+  isMounted
 } from './index.js'
 
 describe('agera', () => {
@@ -1325,7 +1326,7 @@ describe('agera', () => {
       dispose()
     })
 
-    it('should not desync mountable subs count on direct signal read in scope', () => {
+    it('should not change mounted state on direct signal read in scope', () => {
       const $a = mountable(signal(1))
       const log: boolean[] = []
 
@@ -1344,21 +1345,21 @@ describe('agera', () => {
         })
       })
 
-      expect($a.node.subsCount).toBe(2)
+      expect(isMounted($a)).toBe(true)
       expect(log).toEqual([true])
 
       stop()
 
-      expect($a.node.subsCount).toBe(1)
+      expect(isMounted($a)).toBe(true)
       expect(log).toEqual([true])
 
       stopOuter()
 
-      expect($a.node.subsCount).toBe(0)
+      expect(isMounted($a)).toBe(false)
       expect(log).toEqual([true, false])
     })
 
-    it('should decrement effect count on stop', () => {
+    it('should unmount dependencies on scope stop', () => {
       const $a = mountable(signal(1))
       const $b = mountable(signal(1))
       const aListener = vi.fn()
@@ -1380,13 +1381,13 @@ describe('agera', () => {
         })
       })
 
-      expect($a.node.subsCount).toBe(2)
-      expect($b.node.subsCount).toBe(1)
+      expect(isMounted($a)).toBe(true)
+      expect(isMounted($b)).toBe(true)
 
       stop()
 
-      expect($a.node.subsCount).toBe(0)
-      expect($b.node.subsCount).toBe(0)
+      expect(isMounted($a)).toBe(false)
+      expect(isMounted($b)).toBe(false)
     })
   })
 
@@ -1924,7 +1925,7 @@ describe('agera', () => {
       startScope(deferScope(() => { /* flush mounted queue */ }))
 
       expect(callback).not.toHaveBeenCalled()
-      expect($num.node.subsCount).toBe(0)
+      expect(isMounted($num)).toBe(false)
     })
 
     it('should not crash batch flush after scope stop empties its parent', () => {
@@ -1976,7 +1977,7 @@ describe('agera', () => {
 
       expect(cleanupEvents).toEqual(['effect cleanup'])
       expect(mountedEvents).toEqual([])
-      expect($source.node.subsCount).toBe(0)
+      expect(isMounted($source)).toBe(false)
     })
 
     it('should not start linked scope under stopped parent', () => {
@@ -2210,12 +2211,12 @@ describe('agera', () => {
       const observeCallback = vi.fn()
       const stop = observe($num, observeCallback)
 
-      expect($num.node.subsCount).toBe(0)
+      expect(isMounted($num)).toBe(false)
 
       $num(1)
 
       expect(observeCallback).toHaveBeenCalledWith(1)
-      expect($num.node.subsCount).toBe(0)
+      expect(isMounted($num)).toBe(false)
 
       stop()
     })

packages/agera/src/effect.ts

@@ -10,9 +10,9 @@ import {
   boundDeferScope,
   startScope,
   stopScope,
-  untracked
+  untracked,
+  noMount
 } from './internals/system.js'
-import { noMount } from './internals/lifecycle.js'
 
 export {
   effect,

packages/agera/src/index.ts

@@ -5,7 +5,6 @@ export {
   trigger,
   onSignal
 } from './internals/system.js'
-export { noMount } from './internals/lifecycle.js'
 export * from './signal.js'
 export * from './modes.js'
 export * from './effect.js'

packages/agera/src/internals/lifecycle.ts

@@ -12,11 +12,9 @@ export default defineConfig({
     'typescript/no-unsafe-assignment': 'off',
     'typescript/prefer-for-of': 'off',
     'typescript/prefer-optional-chain': 'off',
-    'eslint/no-param-reassign': 'off',
     'eslint/no-label-var': 'off',
     'eslint/no-labels': 'off',
     'eslint/no-constant-condition': 'off',
-    'eslint/no-multi-assign': 'off',
     'eslint/no-sequences': 'off'
   }
 })