Keep the audited public API closed under its own signatures #84

Outcome

Make the audited top-level API closed under its own supported signatures and result objects before M1 closes.

Context

PR #83 removes AssetStore, PackageManifest, and SourceItem from ssrobot.__all__ and describes their modules as implementation details. They nevertheless remain part of supported top-level interfaces:

• dumps() and loads() accept AssetStore, which is required to encode or decode public records containing ArrayValue, including image observations.
• RobotPackage.manifest is a PackageManifest.
• RobotPackage.items and PackageReport.items contain SourceItem values.

Consequently a user cannot fully type or use the documented public API without importing from modules that the contract says are unsupported. Whether a returned value is usually read rather than constructed does not remove its type from the public contract.

Scope

• Restore these names at the top level, which is the smallest compatible resolution, or redesign the public interfaces so they no longer expose them.
• Update the audit decision, public-name count, and public-surface table to match the chosen boundary.
• Keep the checked-in wire schemas unchanged unless the redesign deliberately changes a wire form.

Acceptance criteria

• Every type explicitly accepted, returned, or exposed as a field by the supported top-level API is itself supported at the top level or through a documented stable public module.
• An Observation containing an ArrayValue can be round-tripped through the supported dumps() and loads() API without importing an implementation module.
• The supported way to inspect RobotPackage.manifest, RobotPackage.items, and PackageReport.items does not require an implementation-module import.
• The audit record and generated import report agree on the resulting public names and count.

Found while reviewing PR #83.