fix(ios): runtime clang builds must not use -Werror; cache the fold helper build

[thymikee]

Purpose

Two runtime clang builds compile with -Werror: packages/platform-apple/src/snapshot-source/cache.ts:101 (the AX bridge) and packages/platform-apple/src/foldable/simulator-hid.ts:28 (fold). A new warning in a future Xcode SDK would break these features on users' machines. The code itself hasn't changed, and users can't fix it.

Fold also recompiles apple/fold-helper/Fold.m into a temp directory on every fold call (simulator-hid.ts:19-38; the temp directory is deleted at :66-68). The bridge already has a content-hashed, locked build cache (cache.ts:53-78).

Line references are at 01328de411.

Required behavior

• Runtime builds do not use -Werror. Warnings still fail the repo's own CI checks, if such a check exists or is added. Record which one in the PR.
• The fold helper is built through the same cache mechanism as the bridge. The cache key includes the source hash and the toolchain identity (cache.ts:54,138), so switching DEVELOPER_DIR never serves a stale binary.

Completion conditions

• Unit tests assert that neither argv contains -Werror, and that a second fold call does not invoke clang.
• pnpm check:affected --run is green.