docs(release): the 10.4.0 entry, the index-health concept doc, and the API surfaces — written from the tree, not the plan

docs/PERFORMANCE.md

@@ -323,58 +323,24 @@ Only the graph adjacency index carries a committed scale assertion:
 - ✅ **Single-Node by Design**: One process owns one `path`; scale out at the service layer
 - ✅ **Zero Stubs**: Every line of code is production-ready
 
-## Lazy Loading Performance
+## Index Build at Open (10.4+)
 
-Brainy supports two initialization modes for optimal performance across different use cases:
+As of 10.4, `brain.init()` runs every needed index rebuild to completion before
+it returns — always, regardless of dataset size. There is no lazy,
+first-query rebuild path: a brain either finishes opening healthy, or `init()`
+fails loudly. `disableAutoRebuild` no longer defers index construction to a
+first query; it has no effect on *when* a rebuild runs. Manual control over
+rebuilds is `repairIndex({ rebuild: [...] })`. See
+[Index Health](concepts/index-health.md) for the full read-gate contract
+(providers self-report readiness via `healthReport()`; a read against a
+not-serving provider throws a typed `*NotReadyError` rather than rebuilding
+mid-query).
 
-### Mode 1: Auto-Rebuild (Default)
-
-```javascript
-const brain = new Brainy()
-await brain.init()  // Rebuilds indexes during init (~500ms-3s for 10K entities)
-```
-
-**Performance:**
-- Init time: 500ms-3s (depends on dataset size)
-- First query: Instant (indexes already loaded)
-- Use case: Traditional applications, long-running servers
-
-### Mode 2: Lazy Loading
-
-```javascript
-const brain = new Brainy({ disableAutoRebuild: true })
-await brain.init()  // Returns instantly (0-10ms)
-
-const results = await brain.find({ limit: 10 })  // First query triggers rebuild (~50-200ms)
-const more = await brain.find({ limit: 100 })    // Subsequent queries instant (0ms check)
-```
-
-**Performance:**
-- Init time: 0-10ms (instant)
-- First query: 50-200ms (includes index rebuild for 1K-10K entities)
-- Subsequent queries: 0ms check (instant)
-- Concurrent queries: Wait for same rebuild (mutex prevents duplicates)
-
-**Concurrency Safety:**
-```javascript
-// 100 concurrent queries immediately after init
-await brain.init()
-
-const promises = Array.from({ length: 100 }, () =>
-  brain.find({ limit: 10 })
-)
-
-const results = await Promise.all(promises)
-// ✅ Only 1 rebuild triggered (mutex)
-// ✅ All 100 queries return correct results
-// ✅ Total time: ~60ms (not 6000ms!)
-```
-
-**Use Cases for Lazy Loading:**
-- **Serverless/Edge**: Minimize cold start time (0-10ms init)
-- **Development**: Faster restarts during development
-- **Large datasets**: Defer index loading until needed
-- **Read-heavy workloads**: Writes don't wait for index rebuild
+<!-- The pre-10.4 "Mode 2: Lazy Loading on First Query" section previously
+     documented here (disableAutoRebuild deferring index construction to the
+     first find() call) described a real, now-retired code path. Removed
+     rather than left to mislead; the concept doc above is the current
+     contract. -->
 
 ## Zero Configuration Required
 
@@ -384,10 +350,6 @@ Brainy is designed to be **smart enough to tune itself dynamically**. No configu
 // That's it. Brainy handles everything.
 const brain = new Brainy()
 await brain.init()
-
-// Or with lazy loading for serverless
-const brain = new Brainy({ disableAutoRebuild: true })
-await brain.init()  // Instant (0-10ms)
 ```
 
 ### Automatic Self-Tuning
@@ -395,7 +357,6 @@ await brain.init()  // Instant (0-10ms)
 - **Metadata Index**: Auto-builds sorted indices for range queries on first use
 - **Graph Index**: Auto-flushes every 30 seconds
 - **Default Tuning**: Research-based vector index defaults
-- **Lazy Loading**: Indices built only when needed
 - **Cache Management**: LRU caches with TTL
 
 ### Intelligent Defaults