fix(core): keep the caret in place when backspace has nothing to delete

[myromartsin]

Summary

Backspace with no digit before the caret now stores the caller's selection in history, matching the caret-at-zero guard in the same method. Both input controllers rendered the previous edit's caret through currentState in that branch, which sent the DOM caret to the end of the field when backspacing on a fixed plus. Regression tests cover the fixed-plus and national leading-parenthesis cases.

Motivation

Closes #112.

Conformance impact

None.

Checklist

• Tests added or updated (or a rationale for why none)
• pnpm test, pnpm lint, pnpm format pass locally
• Docs reflect the change (per CONTRIBUTING.md)
• Commit message follows the project convention

[codspeed-hq]

Merging this PR will not alter performance

✅ 39 untouched benchmarks

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_{Comparing fix/digit-less-backspace-caret (44896d5) with main (4c35096)}