nested Timepoint/Duration in struct args rejected by from_json

[yigitcangokmen]

Problem

When a contract function takes a struct with a `Timepoint` or `Duration` field, `stellar contract invoke` fails to parse the JSON argument. Top-level Timepoint/Duration args work fine.

Root cause

Two bugs in `cmd/crates/soroban-spec-tools/src/lib.rs`:

1. `from_json` does not route Timepoint/Duration to primitive parsing

The match arm at line ~328 that dispatches to `from_json_primitives` lists:
`Bool, U128, I128, U256, I256, I32, I64, U32, U64, String, Symbol, Address, MuxedAddress, Bytes, BytesN`

`Timepoint` and `Duration` are missing. When a struct field has type `Timepoint`, `parse_strukt` calls `from_json(Value::Number(1234), ScType::Timepoint)`. Since Timepoint is not in the list, it falls through to the serde catch-all which tries to deserialize a bare JSON number as `ScVal` -- this fails because `ScVal` expects tagged format (`{"timepoint": 1234}`).

Top-level args work because `from_string` pre-wraps the value as `{"timepoint": "value"}` before calling `from_json`.

2. `from_json_primitives` constructs `ScVal::U64` instead of `ScVal::Timepoint`

Line ~937:
```rust
(ScType::U64 | ScType::Timepoint | ScType::Duration, Value::Number(n)) => ScVal::U64(...)
```

This produces `ScVal::U64` for all three types. The Soroban VM distinguishes `SCV_U64` from `SCV_TIMEPOINT` and `SCV_DURATION` -- passing the wrong discriminant causes a host argument type mismatch at runtime.

Reproduce

Contract:
```rust
#[contracttype]
pub struct TimedAction {
pub deadline: Timepoint,
pub name: Symbol,
}

pub fn schedule(env: Env, action: TimedAction) -> TimedAction { action }
```

Invoke:
```
stellar contract invoke --id -- schedule --action '{"deadline": 1760501234, "name": "test"}'
```

Fails with a serde deserialization error on the `deadline` field.

Fix

1. Add `| ScType::Timepoint | ScType::Duration` to the `from_json` primitives routing arm
2. Split the `from_json_primitives` match so Timepoint produces `ScVal::Timepoint(TimePoint(...))` and Duration produces `ScVal::Duration(Duration(...))`