Introduce a singleton AMM configuration account, a PDA derived from the
constant "CONFIG" seed, created once via a new `Initialize` instruction.
The config stores the Token Program ID the AMM issues every chained call
to, replacing the previous behavior of trusting the program owner of a
caller-supplied holding.

The config account's existence is the Program's initialization gate: the
chained-call instructions (new_definition, add_liquidity, remove_liquidity,
swap_exact_input, swap_exact_output) now take the config as their first
account, validate it against `compute_config_pda(self_program_id)`, and
read the Token Program ID from it on demand — rejecting calls until the
Program is initialized. Vaults and user holdings are asserted to match the
configured Token Program. sync_reserves is left ungated, as it cannot act
on a pool that could not have existed before initialization.

- amm_core: AmmConfig type, compute_config_pda/_seed, Initialize variant
- amm: initialize.rs + config threading through chained-call instructions
- guest: initialize instruction; config + self_program_id on gated calls
- tests: config fixtures, init-gate unit tests, end-to-end Initialize VM test