feat(liquidity): add incremental top_up_pair_liquidity

[mxrtins04]

Summary

Adds top_up_pair_liquidity(caller, source, destination, delta) as an additive counterpart to set_pair_liquidity. compute_route_fee debits PairLiquidity via saturating_sub, but the only existing restore path (set_pair_liquidity) overwrites the value absolutely, so a stale oracle read can erase concurrent debits. This entrypoint increments instead of overwriting.

Changes

• src/lib.rs: new top_up_pair_liquidity(env: Env, caller: Address, source: Symbol, destination: Symbol, delta: i128) entrypoint, dual admin-or-oracle auth, AmountMustBePositive on non-positive delta, saturating_add accumulation, i128::MAX sentinel preserved, inline tests
• README.md: added entrypoint to error lists, registration-first invariant lists, paused sweep tables, liquidity consumption section, event reference
• docs/abi.md: documented signature, params, auth rules, errors, event emissions
• docs/roles.md: documented dual-auth policy and oracle responsibility

Testing

• Inline tests covering: successful increment (0 to 500 to 800), non-positive delta rejection (0 and negative), unauthorized caller rejection, oracle-authorized success, unregistered pair rejection, i128::MAX sentinel preservation, saturation to i128::MAX on overflow, net value after saturating_sub debit followed by top-up
• Paused sweep tests confirming top_up_pair_liquidity succeeds while paused under both admin and oracle paths, consistent with other config setters
• cargo fmt --all -- --check, cargo build, cargo test all pass
• Minimum 95% coverage on impacted modules

Security / rollback notes

• No change to existing set_pair_liquidity or compute_route_fee behavior; purely additive
• Auth path identical to existing admin-or-oracle check, no new privilege surface
• Requires pair to be registered (require_pair_registered), consistent with other liquidity operations
• Rollback: revert this PR's commit(s); no migration or storage schema change involved

Closes #201

[mikewheeleer]

@mxrtins04 solid and well-scoped, merging now 🚢