docs: correct four developer-reference claims the source contradicts

Miguel's three P2s and Rames' one finding on #2974, all verified in source
before changing anything.

**`render --json` is not a progress stream.** It prints exactly one
`batch-complete` document at the end (`batchRender.ts:408-418`), asserted as a
single `console.log` in `batchRender.test.ts`. Described as a final result now.

**The iframe drag example never captured the pointer.** `event.target` comes
from `iframe.contentDocument`, so `instanceof Element` against this window's
constructor is always false for a cross-realm node and `setPointerCapture()`
never ran — a pointer leaving the frame then loses `pointerup` and drag state
sticks. Structural feature detection instead, with the reason in a comment so it
does not get "simplified" back.

**The preview adapter example did not compile under strict TypeScript.** `comp`
was captured by the callback before definite assignment (TS2454). Optional, with
`comp?.dispatch(op)`.

**`ORIGIN_APPLY_PATCHES` was imported in a fence that did not use it and used in
fences that did not import it.** Imports do not cross fences, so both examples
were wrong in opposite directions. Rames found the pair in
`open-composition.mdx`; the same shape is in `composition.mdx:630`, which he did
not name. All three fences are self-contained now.

**And `types.mdx` claimed coverage it does not have.** It promised "every type
exported from `@hyperframes/sdk`" while omitting 13 of 42. Eleven are documented
on sibling pages, so the sentence now points at those instead of overclaiming.
The two with no home anywhere — `CompositionVariableType` and
`VariableUsageScan`, both re-exported from the barrel — have entries. The second
is worth having written down: `scanIncomplete` means `usedIds` is a lower bound,
so an id missing from it is unknown rather than unused.

docs/sdk/guides/canvas-integration.mdx

@@ -123,8 +123,11 @@ iframe.addEventListener("load", () => {
     targetId = hit.id;
     startX = event.clientX;
     startY = event.clientY;
-    if (event.target instanceof Element && "setPointerCapture" in event.target) {
-      event.target.setPointerCapture(event.pointerId);
+    // The target comes from the iframe's realm, so `instanceof Element` against
+    // this window's constructor is always false and capture would be skipped —
+    // then a pointer leaving the frame loses `pointerup` and drag state sticks.
+    if (event.target && "setPointerCapture" in event.target) {
+      (event.target as Element).setPointerCapture(event.pointerId);
     }
   });