fix(base): normalize exhausted string prefixes

[oxura]

Fixes #1640.

Once a prefix has been consumed, String.starts_with should not need to inspect the remaining haystack just to return True. The old haystack-first recursion left generic empty-prefix and known-prefix laws stuck.

This puts the internal recursion in pattern-first order while keeping the public (s, p) argument order. A separate internal go is needed because Bend does not allow matching an earlier binder after a later one. The old internal helper is removed, not aliased.

The new test checks empty prefixes, one- and two-character prefixes over arbitrary tails, and an early mismatch over arbitrary tails. The same empty-prefix program fails --check-only on e5527d5 and passes after this change. This is a normalization improvement, not a soundness fix. AI-assisted.

Checked with the independent Lean 4.34 kernel: both the new generic prefix laws and the existing string_contains laws report ALL PROOFS CHECK. The new program also runs on C and JS; the existing string-kit program keeps all 36 expected outputs on both targets.