feat: self-host compositional math helpers

[vycdev2]

Summary

• implement math.is_even, math.is_odd, math.sign, math.to_radians, and math.to_degrees as exported compiler-shipped .jett functions
• remove the five dedicated typechecker signatures and interpreter dispatch arms while retaining math.mod and math.pi as primitive kernels
• cover negative and zero parity cases, assert query signatures resolve to stdlib/math.jett, and rename generic fixtures that previously occupied the now compiler-owned math namespace

Verification

• cargo test -q -p jett_driver math_ — passed: 1 unit test and 13 matching fixture tests
• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed: full workspace suite
• git diff --check — passed

Risk

• Low: the source implementations preserve the existing signatures and arithmetic, the namespace fixture renames are test-only, and focused plus full workspace tests pass.

Closes #28

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

[vycdev]

The implementation and regression coverage look good, but this closes #28 without recording the completed extraction in the project docs. AGENTS.md requires docs/design.md and docs/architecture.md to stay aligned with stdlib changes, and after PR #29 the progress rows will still describe this work as merely “tracked by #28.” Please update the relevant progress/design documentation to say which math helpers are now source-defined in stdlib/math.jett and which remain primitive Rust kernels.

[vycdev2]

Addressed the stdlib documentation feedback. docs/design.md, docs/architecture.md, and docs/progress.md now identify is_even, is_odd, sign, to_radians, and to_degrees as source-defined in stdlib/math.jett; math.mod, math.pi, and the other currently supported math builtins are documented as retained Rust-backed kernels/operations.

Rebased the topic branch onto current upstream/main so the progress updates include the tracking documentation from PR #29.

Verification: cargo fmt --check, cargo test -q -p jett_driver math_, cargo build, cargo test -q, and git diff --check all passed. Head: a67ebb7.

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