refactor(core): introduce SequenceExpr::number() constructor

[hippietrail]

Issues

N/A

Description

Just introduces a number() constructor to SequenceExpr and uses it in the four places that already used SequenceExpr::default().then_number()

How Has This Been Tested?

cargo test

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.