fcp xml invalid start time for clip.

Seems to be a bug in the code that calculates the start time..

start time doesn't match premiere, which is 1002

```
[
{
"OTIO_SCHEMA": "Clip.1",
"effects": [],
"markers": [],
"media_reference": {
"OTIO_SCHEMA": "ExternalReference.1",
"available_range": {
"OTIO_SCHEMA": "TimeRange.1",
"duration": {
"OTIO_SCHEMA": "RationalTime.1",
"rate": 29.97,
"value": 5744
},
"start_time": {
"OTIO_SCHEMA": "RationalTime.1",
"rate": 29.97,
"value": 500
}
},
"name": null,
"target_url": "file://localhost/test.wav"
},
"metadata": {},
"name": "test.wav",
"source_range": {
"OTIO_SCHEMA": "TimeRange.1",
"duration": {
"OTIO_SCHEMA": "RationalTime.1",
"rate": 60.0,
"value": 2246
},
"start_time": {
"OTIO_SCHEMA": "RationalTime.1",
"rate": 60.0,
"value": 1001.001001001001
}
}
}
]

```
```
clipitem id="clipitem-732" premiereChannelType="stereo">
masterclip-81
hw_seq_full_mix.wav
TRUE
11500

60
FALSE

0
2246
0
2246
0
9508665600000

test.wav
file://localhost/test.wav

30
TRUE

5744


30
TRUE

00;00;16;20
500
DF




16
48000

2




audio
1

```

Start with the FCP XML and OTIO JSON examples in this issue, tracing how the clip's source-range start time is calculated and converted between rates. Reproduce the 60 fps/29.97 fps case, compare the generated value with Premiere's expected 1002, and add a regression test once the responsible adapter entry point is located.

Written by the indexing model from the issue text.