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

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

@@ -28,7 +28,7 @@ In Miden, assets serve as the primary means of expressing and transferring value
 All data structures following the Miden asset model that can be exchanged.
 :::
 
-Native assets adhere to the Miden `Asset` model (encoding, issuance, storage). Every native `Asset` is encoded using 32 bytes, including both the [ID](./account/id) of the issuing account and the `Asset` details.
+Native assets adhere to the Miden `Asset` model (encoding, issuance, storage). In an account vault each native `Asset` is stored as an `ASSET_KEY` plus an `ASSET_VALUE` (two words / 64 bytes) that together carry the issuing faucet [ID](./account/id), callback metadata, and asset payload — see [Encoding](#encoding).
 
 ### Issuance
 
@@ -46,11 +46,11 @@ Faucets can issue either fungible or non-fungible assets as defined at account c
 
 #### Fungible asset
 
-Fungible assets are encoded with the amount and the `faucet_id` of the issuing faucet. The amount is always $2^{63}-1$ or smaller, representing the maximum supply for any fungible `Asset`. Examples include ETH and various stablecoins (e.g., DAI, USDT, USDC).
+Fungible assets are encoded with the amount, the issuing `faucet_id`, and the per-asset callback flag (see [Encoding](#encoding) and [Callbacks](#callbacks)). Units merge in a vault only when both the faucet id **and** the callback flag match; the amount is always $2^{63}-2^{31}$ or smaller. Examples include ETH and various stablecoins (e.g., DAI, USDT, USDC).
 
 #### Non-fungible asset
 
-Non-fungible assets are encoded by hashing the `Asset` data into 32 bytes and placing the `faucet_id` as the second element. Examples include NFTs like a DevCon ticket.
+Non-fungible assets are encoded from a 4-limb `DATA_HASH` of the NFT payload plus the issuing `faucet_id` and callback metadata (see [Encoding](#encoding)). Each NFT has a distinct vault key derived from `DATA_HASH[0..2]`; that does **not** imply a distinct SMT leaf — NFTs from the same faucet can share a leaf. Examples include NFTs like a DevCon ticket.
 
 ### Storage
 
@@ -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 | Units that share the same faucet id **and** callback flag occupy one vault entry and merge by amount. Different callback flags are distinct assets (see [Callbacks](#callbacks)). |
+| Non-fungible | taken from `DATA_HASH[0..2]` | Each NFT has a distinct vault key. Distinct keys from the same faucet may still share an SMT leaf — leaf identity is not 1:1 with vault keys. |
+
+### `ASSET_VALUE` layout
+
+| Kind | Value word |
+| --- | --- |
+| Fungible | `[amount, 0, 0, 0]` — `amount ≤ 2⁶³−2³¹` |
+| 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     |