feat(namespace): write-door forgery refusal (user metadata keys may never start 'system.') + refusal messages name both spellings in every branch (the non-colliding case marks system.<f> honestly as NOT valid) — cross-engine message pin alignment

src/db/fieldAddressing.ts

@@ -247,7 +247,8 @@ export function buildUnresolvableMessage(
   return (
     `no metadata field '${raw}' on this store — nothing carries it, so an ordered or ` +
     `filtered read against it cannot mean anything. Spell it metadata.${raw} once the ` +
-    `field exists, or check the field name.`
+    `field exists, or check the field name (system.${raw} is NOT valid — '${raw}' is ` +
+    `not one of the engine's system scalars).`
   )
 }
 

src/utils/paramValidation.ts

@@ -518,7 +518,28 @@ export function validateFindParams(params: FindParams): void {
 /**
  * Validate add parameters
  */
+
+/**
+ * The namespace cannot be forged: a USER metadata key literally spelled
+ * 'system.<anything>' would collide with the engine's explicit address
+ * namespace at read time — refuse it at the write door, loudly, with the
+ * fix in the message (sealed 2026-08-03).
+ */
+function rejectForgedSystemKeys(metadata: Record<string, unknown> | undefined, site: string): void {
+  if (!metadata) return
+  for (const key of Object.keys(metadata)) {
+    if (key.startsWith('system.')) {
+      throw new Error(
+        `${site}: metadata key '${key}' is not allowed — the 'system.' prefix is the ` +
+          `engine's explicit address namespace and cannot be used as a user field name. ` +
+          `Rename the field (e.g. '${key.slice('system.'.length)}').`
+      )
+    }
+  }
+}
+
 export function validateAddParams(params: AddParams): void {
+  rejectForgedSystemKeys(params.metadata as Record<string, unknown> | undefined, 'add()')
   // Universal truth: must have data or vector
   if (!params.data && !params.vector) {
     throw new Error(
@@ -559,6 +580,7 @@ export function validateAddParams(params: AddParams): void {
  * Validate update parameters
  */
 export function validateUpdateParams(params: UpdateParams): void {
+  rejectForgedSystemKeys(params.metadata as Record<string, unknown> | undefined, 'update()')
   // Universal truth: must have an ID
   if (!params.id) {
     throw new Error('id is required for update')