refactor(8.0): add config.vector path + 'vectors' cache category (scaffold step 2)

Step 2 of the brainy 8.0 rename scaffolding. Adds the new algorithm-neutral
config surface alongside the legacy config.hnsw path (which becomes a compat
shim removed in the final cleanup commit).

CHANGES

src/utils/recallPreset.ts (NEW)
- DEFAULT_RECALL = 'balanced'
- HNSW_PRESETS table — fast/balanced/accurate → (M, efConstruction, efSearch).
  'balanced' equals brainy 7.x DEFAULT_CONFIG byte-for-byte so a 7.x → 8.0
  upgrade with no explicit recall value is a no-op.
- resolveRecallHnswKnobs(preset) — preset → knobs.
- resolveJsHnswConfig(vectorConfig) — preset first, then `advanced.hnsw` overrides
  win. Explicit knobs always beat the preset.

src/types/brainy.types.ts
- New optional `vector` field on BrainyConfig:
    vector?: {
      recall?: 'fast' | 'balanced' | 'accurate'
      quantization?: { enabled?, bits?: 8 | 4, rerankMultiplier? }
      vectorStorage?: 'memory' | 'lazy'
      advanced?: {
        hnsw?: { M?, efConstruction?, efSearch?, ml? }
        diskann?: { pqM?, pqKsub?, maxDegree?, searchListSize?, alpha?, ... }
      }
    }
- `hnsw` field marked @deprecated with note that it stays as a compat shim
  during the 8.0 rename and is removed in the final cleanup commit.

src/utils/unifiedCache.ts
- Cache-category union widened from
    'hnsw' | 'metadata' | 'embedding' | 'other'
  to
    'hnsw' | 'vectors' | 'metadata' | 'embedding' | 'other'
- 5 union sites + 4 typed-counter maps (typeAccessCounts, checkFairness
  typeSizes/typeCounts/accessRatios/sizeRatios, getStats typeSizes/typeCounts,
  evictType loop) all gained the 'vectors' key with initial value 0.
- The hard-coded fairness-check loop now iterates both keys.
- Final 8.0 cleanup will narrow back to 'vectors' | 'metadata' | 'embedding' | 'other'.

src/brainy.ts (normalizeConfig)
- Adds the new `vector` field to the Required<BrainyConfig> return shape so
  TS compiles. Reads config?.vector ?? undefined as any (same pattern as
  the other optional config fields).

NO-OP scope

No behavioural change. The new config.vector path is silently ignored at
runtime in this commit — wired up in step 4 (storage + index resolution).
Pure surface plumbing. Tests + build green.

VERIFICATION

- npx tsc --noEmit: clean
- npm test: 1468 / 1468 unit

src/utils/recallPreset.ts

@@ -0,0 +1,91 @@
+/**
+ * @module utils/recallPreset
+ * @description Maps Brainy 8.0's algorithm-neutral `recall` preset to the
+ * underlying index-specific knobs. The same user-facing word (`'fast'` /
+ * `'balanced'` / `'accurate'`) means "the right thing happens" whether
+ * Brainy's open-core JS HNSW path is in play or a native acceleration
+ * provider (DiskANN-style) has taken over the `'vector'` provider key.
+ *
+ * **Public contract** — locked in handoff thread BRAINY-8.0-RENAME-COORDINATION
+ * § A.2 (cortex-confirmed 2026-06-09):
+ * - `'fast'` — minimum-latency search, accepts lower recall
+ * - `'balanced'` — Brainy 7.x defaults, the right pick for almost everyone
+ * - `'accurate'` — maximum recall, accepts higher latency
+ *
+ * Default when `recall` is omitted: `'balanced'`.
+ *
+ * **Knob mapping** — the JS HNSW preset values below match what Brainy 7.x
+ * shipped as defaults for `'balanced'`. Native acceleration providers (e.g.
+ * cortex's DiskANN wrapper) read the same `recall` value off the index
+ * config and translate it to their own internal knobs.
+ */
+
+import type { HNSWConfig } from '../coreTypes.js'
+
+export type RecallPreset = 'fast' | 'balanced' | 'accurate'
+
+/**
+ * The default preset when `config.vector.recall` is omitted. Matches Brainy
+ * 7.x's historical HNSW defaults exactly (`M=16`, `efConstruction=200`,
+ * `efSearch=50`).
+ */
+export const DEFAULT_RECALL: RecallPreset = 'balanced'
+
+/**
+ * The set of HNSW knobs that the `recall` preset controls. Power users may
+ * override any of these via `config.vector.advanced.hnsw` — the preset is the
+ * floor, explicit overrides win.
+ */
+export interface RecallHnswKnobs {
+  M: number
+  efConstruction: number
+  efSearch: number
+}
+
+/**
+ * Preset → JS HNSW knob tuples. The `'balanced'` entry equals Brainy 7.x's
+ * `DEFAULT_CONFIG` so a 7.x → 8.0 upgrade with no explicit `recall` value is
+ * a no-op.
+ */
+const HNSW_PRESETS: Readonly<Record<RecallPreset, Readonly<RecallHnswKnobs>>> = {
+  fast:     { M: 16, efConstruction: 100, efSearch: 30  },
+  balanced: { M: 16, efConstruction: 200, efSearch: 50  },
+  accurate: { M: 32, efConstruction: 400, efSearch: 100 },
+}
+
+/**
+ * Resolve a preset name (or `undefined` for the default) to its underlying
+ * HNSW knob tuple.
+ *
+ * @param preset - The preset name, or `undefined` to use {@link DEFAULT_RECALL}.
+ * @returns The HNSW knob values for the preset.
+ */
+export function resolveRecallHnswKnobs(preset: RecallPreset | undefined): RecallHnswKnobs {
+  return HNSW_PRESETS[preset ?? DEFAULT_RECALL]
+}
+
+/**
+ * Resolve a Brainy 8.0 `VectorIndexConfig` into a concrete `HNSWConfig` for
+ * the open-core JS HNSW path. Applies the recall preset first, then layers
+ * the optional `advanced.hnsw` overrides so explicit knobs always win.
+ *
+ * Native acceleration providers do NOT use this function — they read
+ * `config.vector.recall` directly and apply their own preset table. This
+ * helper exists for Brainy's own JS HNSW implementation only.
+ *
+ * @param vectorConfig - The user-supplied `config.vector` block. May be omitted.
+ * @returns A complete `HNSWConfig` with `M`, `efConstruction`, `efSearch`, `ml`
+ *   populated from preset + overrides.
+ */
+export function resolveJsHnswConfig(
+  vectorConfig?: { recall?: RecallPreset; advanced?: { hnsw?: Partial<HNSWConfig> } }
+): Pick<HNSWConfig, 'M' | 'efConstruction' | 'efSearch' | 'ml'> {
+  const preset = resolveRecallHnswKnobs(vectorConfig?.recall)
+  const overrides = vectorConfig?.advanced?.hnsw ?? {}
+  return {
+    M: overrides.M ?? preset.M,
+    efConstruction: overrides.efConstruction ?? preset.efConstruction,
+    efSearch: overrides.efSearch ?? preset.efSearch,
+    ml: overrides.ml ?? 16,
+  }
+}