Expose the deployed product dotNS identifier to products

[replghost]

Products currently need to know their own DotNsIdentifier to call APIs such as host_account_get, where ProductAccountId = (DotNsIdentifier, DerivationIndex). The host already knows the calling product's identity for scoping permissions, entropy, storage, and allowances, but there does not appear to be a product-facing way for the sandbox to learn the dotNS name it was actually deployed under.

This creates a mismatch: a product bundle may be deployed under different dotNS names, but account derivation and product-scoped APIs depend on the canonical deployed name, not a hardcoded value inside the product.

Relevant references:

• docs/design/host-api-protocol.md defines host_handshake as only returning Result<(), HandshakeErr>.
• src/data/v02/types.ts documents host_account_get as requiring the product to provide ProductAccountId.
• src/data/v02/types.ts defines DotNsIdentifier.

Proposal:

Add a product-facing way to get the runtime product identity, for example:

type ProductIdentity = Struct({
  dotNsId: DotNsIdentifier,
});

fn host_product_identity_get() -> Result<ProductIdentity, GenericErr>;

Alternatively, include this in the successful handshake response.

Expected behavior:

• The value is supplied by the host from the deployed/resolved product identity.
• It matches the identity used by the host for product-scoped permissions, entropy derivation, local storage namespacing, product accounts, and allowance scoping.
• It is available after handshake and does not require user authentication, since it reveals the product's own identity rather than user data.
• Product SDKs expose a simple helper, e.g. truApi.getProductIdentity() or truApi.getProductDotNsId().

Acceptance criteria:

• Spec types and docs define how a product obtains its deployed dotNS identifier.
• SDK exposes the value to products.
• Host implementations return the deployed dotNS name used for scoping.
• Product account examples no longer require hardcoding "my-product.dot" when the product wants its own account.