Replace into_inner() with into() for nutype domain types

[jwilger]

Background

Currently we call .into_inner() on nutype domain types when we need the wrapped primitive value. This is verbose and non-idiomatic. Rust's Into trait is the standard way to perform type conversions.

Changes

1. Find all locations where .into_inner() is called on nutype types
2. Implement Into<T> for each nutype domain type (BatchSize, StreamPosition, StreamVersion, etc.)
3. Replace .into_inner() calls with .into()
4. Ensure tests pass and code compiles

Example

// Current (verbose)
let size: usize = batch_size.into_inner();

// After (idiomatic)
let size: usize = batch_size.into();

Acceptance Criteria

• Into<T> implemented for all nutype domain types
• All .into_inner() calls replaced with .into()
• Existing test suite passes unchanged
• More idiomatic Rust code

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Migrated from beads issue: eventcore-4i0