fix(reads): the read gate is per-family; a write carrying unchanged data never re-embeds

Two cures from the pair's first production adoption, both measured live.

THE READ GATE IS PER-FAMILY. The report-driven gate refused on ANY
provider's not-ready verdict at every read choke point — so a pure
metadata find({ where }) was refused because the VECTOR leg was not
serving, and a deployment's badge reads returned errors for a verdict that
had nothing to do with them. A read may only be refused by the family it
actually consults: metadata reads by the metadata leg (plus graph for a
`connected` filter), vector search by the vector leg, traversal by the
graph leg. Callers name what they need; the existing narration-once-per-
generation and typed-refusal laws are unchanged within a family.

NO RE-EMBED ON UNCHANGED DATA. update() — and its transact() planner —
treated any write that carried `data` as a data change: with
deferEmbedding it queued a landing, and the worker re-embedded and re-landed
a vector for content that had not changed. A host heartbeat re-writing an
unchanged row every few seconds therefore fed a live index-row loop on a
production store. A write carrying the row's current data (structural
compare, key order normalized) is now not a data change: no re-embed, no
deferred landing, no vector rewrite; the metadata write itself still
commits. A real change re-embeds exactly as before.

Pinned in tests/integration/read-gate-scope-and-no-reembed.test.ts — both
pins red-proved against the unfixed code with the production shapes
verbatim. Two health-gate pins that encoded the old brain-global scope are
re-pointed to the family their reads consult.

tests/integration/health-gate.test.ts

@@ -186,6 +186,8 @@ describe('health gate (b) — unledgered is unknown: never blocks a serving prov
 })
 
 describe('health gate (c) — degraded-but-serving narrates once per generation', () => {
+  // PER-FAMILY LAW (10.4.1): a metadata find() consults the METADATA leg only — the
+  // degraded report lives on the family the read actually consults.
   it('a heal:"repair" failure serves; narrates once per generation, twice across a generation bump', async () => {
     const brain = new Brainy(createTestConfig({ silent: true }))
     await brain.init()
@@ -195,7 +197,7 @@ describe('health gate (c) — degraded-but-serving narrates once per generation'
 
     const internals = internalsOf(brain)
     let generation = 1
-    internals.index.healthReport = () =>
+    internals.metadataIndex.healthReport = () =>
       healthReport({
         provider: 'vector',
         serving: true,
@@ -216,7 +218,7 @@ describe('health gate (c) — degraded-but-serving narrates once per generation'
     await expect(brain.find({ where: { team: 'atlas' } })).resolves.toHaveLength(1)
     expect(countNarrations()).toBe(2) // generation bumped — a second narration
 
-    delete internals.index.healthReport
+    delete internals.metadataIndex.healthReport
   })
 })
 
@@ -332,6 +334,7 @@ describe('health gate (f) — the ceremony door: explicit rebuild bypasses invar
 })
 
 describe('health gate (g) — a throwing healthReport() is a contract violation, never read as healthy', () => {
+  // PER-FAMILY LAW (10.4.1): the throwing report sits on the family the read consults.
   it('healthReport() that throws refuses loudly with the typed NotReady error naming the throw', async () => {
     const brain = new Brainy(createTestConfig({ silent: true }))
     await brain.init()
@@ -340,13 +343,13 @@ describe('health gate (g) — a throwing healthReport() is a contract violation,
     await brain.flush()
 
     const internals = internalsOf(brain)
-    internals.index.healthReport = () => {
+    internals.metadataIndex.healthReport = () => {
       throw new Error('accelerator: mmap window busy')
     }
 
-    await expect(brain.find({ where: { team: 'atlas' } })).rejects.toBeInstanceOf(VectorIndexNotReadyError)
+    await expect(brain.find({ where: { team: 'atlas' } })).rejects.toBeInstanceOf(MetadataIndexNotReadyError)
     await expect(brain.find({ where: { team: 'atlas' } })).rejects.toThrow(/mmap window busy/)
 
-    delete internals.index.healthReport
+    delete internals.metadataIndex.healthReport
   })
 })