ci(ios): re-home the dispatch-only iOS workflows to the repo root

[TarikGul]

GitHub only reads workflows from the repository root, so everything under
hosts/ios/.github/workflows/ does nothing today. This moves the seven that
never trigger themselves: two workflow_call and five workflow_dispatch.

Picking those seven means no scheduled or pull request behaviour changes. The
mechanical path work lands and gets reviewed now, ahead of the credentials and
runners in #672 that the actual PR checks need.

What moved:

_build_distribute.yml        -> ios-build-distribute.yml
_prepare_pipeline.yml        -> ios-prepare-pipeline.yml
nightly_distribution.yml     -> ios-nightly-distribution.yml
register_device.yml          -> ios-register-device.yml
release_prepare.yml          -> ios-release-prepare.yml
safetynet_distribution.yml   -> ios-safetynet-distribution.yml
testflight_distribution.yml  -> ios-testflight-distribution.yml

Composite actions stay under hosts/ios and are referenced as
./hosts/ios/.github/actions/.... That is also what makes the workflow_call
references between these files work, since a local uses: path always resolves
from the repository root no matter where the calling file lives, so those
references were broken while the files sat in a subdirectory.

Steps that drive the app run from hosts/ios: the fastlane invocations, the
xcconfig edit, the TestFlight build number lookup. Steps that take a path
argument keep a root relative one, so read-build-version gets a prefixed
pbxproj default and a prefixed script path rather than a working directory.

The shared composite actions needed the same treatment. Bundler resolves
hosts/ios/Gemfile, the SPM cache points at hosts/ios/source_packages, the
generated secrets and GoogleService-Info.plist land under hosts/ios, and the
ipa uploads from there.

Worth being clear about what this does not establish. Nothing invokes these
workflows, and a real build needs the credentials in #672, so the checks here
are actionlint plus every local uses: path resolving. Whether the app actually
builds from this layout is unproven until #672 lands. Left as a draft for that
reason.

Part of #680.

[github-actions]

CI Status: 14 required jobs green, 11 passed and 3 skipped by path filter.

All job results

job	result
android-bindings	skipped
changes	success
codegen	success
e2e	skipped
explorer	success
ios-bindings	success
ios-swift	skipped
licenses	success
playground	success
rust	success
ts-client	success
ts-debugger	success
ts-host	success
wasm-provider	success

_{Commit e9408ba0 · run log}