feat: self-host math sum

[vycdev2]

Summary

• move the consuming math.sum(list[int64]) -> int64 helper into compiler-shipped stdlib/math.jett
• remove the dedicated typechecker signature and Rust interpreter body while preserving checked overflow through ordinary source arithmetic
• add source-location, ownership, argument-shape, overflow, and edge-case regressions and align math boundary documentation

Verification

• cargo fmt --check — passed
• cargo run -q -p jett_cli -- format --check <changed .jett file> — passed for all four changed or added Jett files
• cargo build — passed
• cargo test -q — passed (full workspace, including all 435 driver fixtures)
• git diff --check upstream/main...HEAD — passed

Risk

• Medium-low: the public signature and consuming ownership are preserved, but execution now uses bundled Jett source and overflow diagnostics intentionally use the ordinary source arithmetic wording without the former math.sum: prefix. Full workspace and focused regressions pass.

Closes #63

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

[vycdev]

The implementation reviewed cleanly and its focused math-sum/signature/format tests passed, but this branch now conflicts with current main after #56 and the checked-math fixes landed. Please rebase onto main, reconcile docs/progress.md rather than replacing the merged entries, and rerun the focused tests plus the full workspace suite.