feat: self-host compositional string helpers

[vycdev2]

Summary

• define string.is_not_empty, string.reverse, string.after, string.before, and string.between as exported functions in stdlib/string.jett
• preserve missing/empty-marker and grapheme behavior by composing existing runtime-backed string/list kernels
• allow only compiler-shipped namespace string to coexist with the builtin type name, while continuing to reject project declarations
• remove the five hardcoded checker signatures and interpreter bodies, add source-query and edge-case regressions, and align stdlib documentation

Verification

• cargo fmt --check — passed
• cargo run -q -p jett_cli -- format --check <changed .jett file> — passed for stdlib/string.jett, tests/run_pass/string_extra.jett, and tests/run_pass/string_search.jett
• cargo build — passed
• cargo test -q -p jett_driver run_pass_string — passed (7 focused fixtures)
• cargo test -q — passed (full workspace, including 428 driver fixtures)
• git diff --check upstream/main...HEAD — passed

Risk

• Medium-low: public signatures and behavior are preserved, but execution moves from Rust match arms to bundled Jett source and adds a narrow parser/resolver exception for the compiler-shipped string namespace. Full workspace and focused edge-case tests pass.

Closes #55

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.

[vycdev]

Focused review and tests passed, but this branch now conflicts with main after #30 was merged. Please rebase and preserve both the source-defined math-helper and string-helper signature-query tests plus both sets of docs/progress.md updates. After rebasing, rerun cargo fmt --check and the full workspace suite.