fix(amm): require signer on user token holdings in swap and add-liquidity

The swap and add-liquidity instructions debited user-owned token holdings
without requiring those accounts to be signers. Mark them `signer` so a
transaction can't move a user's tokens without their authorization:

- add liquidity: `user_holding_lp` is now `#[account(mut, signer)]`
- swap (both directions): `user_holding_a` and `user_holding_b` are now
  `#[account(mut, signer)]`

Regenerate artifacts/amm-idl.json to reflect the new signer metadata.

Update integration tests accordingly: swaps now sign and supply nonces for
both user holdings (incrementing both nonces), and
`amm_new_definition_precreated_zero_balance_user_lp` becomes
`amm_new_definition_precreated_user_lp_unsigned_fails`, asserting an unsigned
pre-existing LP holding is rejected and the transaction reverts.

programs/amm/methods/guest/src/bin/amm.rs

@@ -163,7 +163,7 @@ mod amm {
         user_holding_a: AccountWithMetadata,
         #[account(mut, signer)]
         user_holding_b: AccountWithMetadata,
-        #[account(mut)]
+        #[account(mut, signer)]
         user_holding_lp: AccountWithMetadata,
         #[account(init)]
         current_tick_account: AccountWithMetadata,
@@ -312,9 +312,9 @@ mod amm {
         vault_a: AccountWithMetadata,
         #[account(mut)]
         vault_b: AccountWithMetadata,
-        #[account(mut)]
+        #[account(mut, signer)]
         user_holding_a: AccountWithMetadata,
-        #[account(mut)]
+        #[account(mut, signer)]
         user_holding_b: AccountWithMetadata,
         #[account(mut)]
         current_tick_account: AccountWithMetadata,
@@ -357,9 +357,9 @@ mod amm {
         vault_a: AccountWithMetadata,
         #[account(mut)]
         vault_b: AccountWithMetadata,
-        #[account(mut)]
+        #[account(mut, signer)]
         user_holding_a: AccountWithMetadata,
-        #[account(mut)]
+        #[account(mut, signer)]
         user_holding_b: AccountWithMetadata,
         #[account(mut)]
         current_tick_account: AccountWithMetadata,