docs(0.14): document native asset vault key/value encoding

versioned_docs/version-0.14/reference/protocol/asset.md

@@ -123,6 +123,49 @@ A callback is not invoked in any of these cases:
 
 This means assets with callbacks enabled can still be used even if the faucet has not (yet) registered a callback procedure.
 
+## Encoding
+
+Every native asset occupies two words (64 bytes): an `ASSET_KEY` (vault key) that identifies the asset in an account vault SMT, and an `ASSET_VALUE` that carries the asset payload.
+
+### `ASSET_KEY` / `AssetVaultKey` layout
+
+The vault key is a [`Word`](https://docs.rs/miden-protocol) with this limb order (matching `AssetVaultKey::to_word()` in v0.14):
+
+```text
+[
+  asset_id_suffix,   // Felt
+  asset_id_prefix,   // Felt
+  faucet_id_suffix_and_metadata,  // Felt
+  faucet_id_prefix   // Felt
+]
+```
+
+Where `faucet_id_suffix_and_metadata` packs:
+
+```text
+[ faucet_id_suffix (56 bits) | 7 zero bits | callbacks_enabled (1 bit) ]
+```
+
+The low byte of the faucet account-id suffix is reserved for metadata (it is zero in a bare faucet id). The least-significant bit of that byte is the [`AssetCallbackFlag`](#callbacks): `0` disabled, `1` enabled.
+
+### Fungible vs non-fungible keys
+
+| Kind | `asset_id_*` limbs | Meaning |
+| --- | --- | --- |
+| Fungible | both zero | All fungible units from a faucet share one vault leaf and merge by amount. |
+| Non-fungible | taken from `DATA_HASH[0..2]` | Each NFT has a distinct key (and therefore a distinct leaf). |
+
+### `ASSET_VALUE` layout
+
+| Kind | Value word |
+| --- | --- |
+| Fungible | `[amount, 0, 0, 0]` — `amount ≤ 2⁶³−1` |
+| Non-fungible | full 4-limb `DATA_HASH` of the NFT payload |
+
+### Stdlib helpers
+
+When building assets from arbitrary code (not as the faucet itself), use `miden::protocol::asset::create_fungible_asset` / `create_non_fungible_asset`. Both take an `enable_callbacks` flag so the metadata bit is set explicitly — see [Protocol library](./protocol_library.md#asset-procedures-midenprotocolasset).
+
 ## Alternative asset models
 
 :::note

versioned_docs/version-0.14/reference/protocol/protocol_library.md

@@ -163,4 +163,4 @@ Asset procedures provide utilities for creating fungible and non-fungible assets
 | Procedure                  | Description                                                                                                                                                          | Context |
 | -------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------- |
 | `create_fungible_asset`     | Builds a fungible asset for the specified fungible faucet and amount.<br/><br/>**Inputs:** `[enable_callbacks, faucet_id_suffix, faucet_id_prefix, amount]`<br/>**Outputs:** `[ASSET_KEY, ASSET_VALUE]` | Any     |
-| `create_non_fungible_asset` | Builds a non-fungible asset for the specified non-fungible faucet and data hash.<br/><br/>**Inputs:** `[faucet_id_suffix, faucet_id_prefix, DATA_HASH]`<br/>**Outputs:** `[ASSET_KEY, ASSET_VALUE]`     | Any     |
+| `create_non_fungible_asset` | Builds a non-fungible asset for the specified non-fungible faucet and data hash.<br/><br/>**Inputs:** `[enable_callbacks, faucet_id_suffix, faucet_id_prefix, DATA_HASH]`<br/>**Outputs:** `[ASSET_KEY, ASSET_VALUE]` | Any     |