feat(stablecoin): add deposit_collateral instruction

[gravityblast]

Adds deposit_collateral (spec §10.5) — the inverse of withdraw_collateral,
letting an owner top up an existing position.

Five accounts, one chained Token::Transfer with no PDA seed: the sender is the
user's own holding, authorized by the witness set. (withdraw_collateral needs
a seed because there the vault is the sender.)

Two deliberate omissions, both per spec:

• No is_frozen check. protocol_parameters is read only for
  collateral_definition_id. §7 keeps deleveraging available in emergencies, so
  a deposit works while frozen — the opposite of open_position. Pinned by a test.
• No clock account and no §6.2 collateralization check. Deposits don't
  reference time and can only improve the position.

14 unit tests: happy path, frozen, zero amount, debt left untouched, both
authorization paths, uninitialized/foreign-owned/wrong-address position, wrong
vault, uninitialized ProtocolParameters, wrong Token Program, wrong collateral
definition, and overflow.

The open/withdraw e2e test gains a deposit step between the two, so the new
instruction is exercised through the zkVM; its withdraw assertions are rebased on
the topped-up balance.

Third of eight issues in Plan 3 (#173). Stacked on #341.

closes #176