feat(stablecoin): implement open_position

Adds the `open_position` instruction to the Stablecoin Program. The instruction
claims a per-owner `Position` PDA, initializes a collateral vault token holding
via a chained `Token::InitializeAccount` under the vault's PDA authority, and
moves `collateral_amount` from the user's holding into the vault with a chained
`Token::Transfer`. `Position` is persisted with `collateral_amount` and
`debt_amount = 0`; the debt path is deferred to `generate_debt`.

- Add `Position` struct, `OpenPosition` instruction variant, and
  `compute_position_pda{,_seed}` / `compute_position_vault_pda{,_seed}` helpers
  in `stablecoin_core` with domain-separated PDA seeds.
- Implement `open_position::open_position` mirroring the ATA `create` and AMM
  `new_definition` patterns: authorization and uninitialized-state asserts, PDA
  verification, and same-transaction chained `InitializeAccount` + `Transfer`.
- Wire the new instruction through the SPEL guest and regenerate the stablecoin
  IDL artifact.
- Cover the happy path, all assertion paths, and PDA determinism /
  non-collision in 11 new unit tests.

artifacts/stablecoin-idl.json

@@ -13,6 +13,51 @@
         }
       ],
       "args": []
+    },
+    {
+      "name": "open_position",
+      "accounts": [
+        {
+          "name": "owner",
+          "writable": false,
+          "signer": false,
+          "init": false
+        },
+        {
+          "name": "position",
+          "writable": false,
+          "signer": false,
+          "init": false
+        },
+        {
+          "name": "vault",
+          "writable": false,
+          "signer": false,
+          "init": false
+        },
+        {
+          "name": "user_holding",
+          "writable": false,
+          "signer": false,
+          "init": false
+        },
+        {
+          "name": "token_definition",
+          "writable": false,
+          "signer": false,
+          "init": false
+        }
+      ],
+      "args": [
+        {
+          "name": "stablecoin_program_id",
+          "type": "program_id"
+        },
+        {
+          "name": "collateral_amount",
+          "type": "u128"
+        }
+      ]
     }
   ],
   "instruction_type": "stablecoin_core::Instruction"