Add end-to-end 15m ES RTH dataset export

[sybapp]

Follow-up from review

The Versioned Dataset Contract accepts 15m, but the collector/export path is still hard-coded around ES RTH 5-minute datasets.

What to build

Add end-to-end support for 15-minute ES RTH datasets as the secondary comparison interval described in the PRD.

Acceptance criteria

• The collector can export ES RTH 15-minute bars from TradingView.
• The exported manifest records bar.interval as 15m and keeps the same symbol/session/continuous futures metadata expectations as the 5m path.
• The dataset contract validator accepts the exported 15m dataset and rejects malformed 15m gaps.
• There is a deterministic fixture or mocked collector test for the 15m export shape.
• The path does not require NautilusTrader to run.

Context

Review finding: the contract validator accepts 15m, but buildEsRth5mDataset / collectEsRth5mDataset and the example command are 5m-specific.

Related PRD story: "I want 15-minute bars supported as a secondary comparison interval."