fix(base): stop prefix checks at the matched prefix

[EpsilonPhoenix]

Fixes #1640.

String.starts_with(s, "") previously stayed stuck when s was unknown. Matching a known prefix also unnecessarily required inspecting the remaining haystack.

Match the prefix first, returning True as soon as it is exhausted. Preserve existing signatures and short-circuit on character mismatches.

Adds proof regressions for arbitrary strings and tails, plus runtime coverage for empty strings, exact matches, longer prefixes, mismatches, and Unicode.

Validation:

• All 36 Base tests match their expected output.
• All five string tests pass in the interpreter, generated JavaScript, and native C.
• BendTT validates the new regression test and existing string_contains and string_kit tests.