fix(find): the projection seam is ES-private, and document the projection

TypeScript's `private` is compile-time only, so the seam's helpers were real
prototype methods and the generated contract manifest listed them as public
DOORS — which would have obliged every other engine to implement an internal
detail. They are `#`-private now and the manifest is unchanged by this branch.

Found while checking that: docs/api-contract.json was ALREADY stale at v10.4.11
— promoteQueuedFlush and startFlushLeader are in src and absent from the
manifest, so they leaked the same way and were never re-emitted. Left alone
here rather than folded into this branch; it is someone's to fix deliberately,
and the fix is the same # conversion.

docs/FIND_SYSTEM.md gains the projection: the rules, why a missing field is
absent rather than an error, where the values come from and what a field the
column cannot serve costs.

docs/FIND_SYSTEM.md

@@ -369,6 +369,71 @@ return results.slice(offset, offset + limit)
 // → Auto-correction: Use most likely alternative based on affinity data
 ```
 
+## Field Projection (`fields`)
+
+`find()` and `get()` accept a `fields` list. Without it they return the whole
+record; with it they return only the fields you name — and, where the index can
+supply them, without opening the canonical record at all.
+
+```ts
+// A list page: two user fields and one engine scalar. No document bodies.
+await brain.find({
+  where: { kind: 'post' },
+  fields: ['title', 'slug', 'system.createdAt'],
+  limit: 50
+})
+
+await brain.get(id, { fields: ['title'] })
+```
+
+### Why it exists
+
+A list view that renders a title and a date does not need the body, but without
+a projection every row hydrates its full record and throws almost all of it
+away. On a posts list that is the dominant cost of the query.
+
+### The rules
+
+| | |
+|---|---|
+| **`fields` absent** | The full record, byte-identical to before. Nothing changes. |
+| **Field names** | The one addressing law: a bare name is user metadata (`'title'`), `system.*` is an engine scalar (`'system.createdAt'`). |
+| **A field the row lacks** | Simply **absent** from the result. Never an error. |
+| **Identity** | Every row keeps its `id` (and `score` on `find`) regardless — a row you cannot identify is not a row. |
+| **Where values come from** | The **column store**, which holds raw values. Never the sparse index, which buckets timestamps for range queries. |
+| **A field the column cannot serve** | The canonical record is read for that field only. Correct, just not free. |
+
+### Missing fields are absent, not errors
+
+This is deliberate and differs from `orderBy`, which throws
+`UnresolvableFieldError` for an unknown field. A typo in `orderBy` silently
+changes the ordering, so it must be loud. A projection asks "give me these if
+you have them", and an optional field must not turn a list into a failure — so
+`fields` uses the permissive path.
+
+### Cost
+
+When every named field is column-served, a projected page performs **zero**
+canonical reads. When one is not, only that read happens and the rest still come
+from the index. Both are pinned by counting reads rather than timing them, in
+`tests/integration/find-fields-projection.test.ts`.
+
+### `related()` takes no `fields`
+
+A `Relation` carries `from` and `to` as **ids** and hydrates no entity record,
+so there is nothing for a projection to trim. Projecting the endpoints would be
+a new capability rather than a projection of an existing one.
+
+### For engine implementers
+
+Projection is served through an optional provider door,
+`getScalarsForIds(ids, fields)` on `MetadataIndexProvider`. The contract is in
+`src/plugin.ts`; the short version is **return only what you can serve exactly,
+and say what you served**. The caller diffs the answer against the request and
+reads records for the remainder, so omission costs a read while a wrong value is
+a wrong answer nobody can see. An engine without the door still works — every
+field falls back to the record.
+
 ## Performance Characteristics
 
 ### Query Performance by Type