Fix nested parallel transition targets

[SandroMaglione]

What changed

• Require complete region selection when target.local or target.branch enters an inactive nested parallel state.
• Preserve partial branch updates for parallel states that are already active.
• Carry complete nested target subtrees through normalization while preserving outer active parallel regions and ancestor values.
• Document the semantics and add a patch changeset so the Changesets release PR (Version Packages #6) updates after merge.

Root cause

Local and branch target builders represented every destination as a single leaf path. TypeScript therefore accepted an incomplete nested parallel target, but runtime snapshot construction later failed because one or more active regions were missing.

Verification

• pnpm check
• 196 runtime tests passed.
• 88 type tests / 337 assertions passed.
• Strict consumer and package checks passed.