enhancement: self-host math.sum in stdlib

[vycdev2]

Summary

Move the monomorphic math.sum(list[int64]) returns int64 helper from its hardcoded checker signature and Rust interpreter body into the exported compiler-shipped .jett math module, while preserving consuming ownership and deterministic overflow behavior.

This is a narrow follow-up to #28 / PR #30. It covers only an aggregate helper whose existing public type is directly expressible in Jett; it does not broaden the closed compiler-owned numeric overload boundary.

Source documentation

• docs/progress.md — Phase D, Core stdlib (.jett files)
• docs/progress.md — Phase K, math
• docs/open_design/math_numeric_overload_boundary.md

Current state

math.sum has a hardcoded list[int64] -> int64 signature in jett_typecheck and an interpreter dispatch arm that loops over a runtime list. The existing run-pass fixture covers a representative non-empty sum. PR #47 separately changes unchecked host accumulation into a deterministic overflow error and adds an int64.MAX + 1 runtime-failure regression.

Unlike math.abs, math.min, and math.max, math.sum does not require the compiler-owned int64/float64 overload table. Its body can be expressed with an int64 accumulator, ordinary list iteration, and checked source-level addition. The current helper consumes its list argument rather than borrowing it; extraction must preserve that ownership behavior.

Scope

Includes:

• add exported math.sum(values: list[int64]) returns int64 source under the compiler-shipped math stdlib module;
• remove only the dedicated hardcoded typechecker signature and Rust interpreter dispatch arm for math.sum;
• preserve an empty-list result of 0, normal and negative accumulation, argument/return diagnostics, consuming ownership, and deterministic overflow reporting;
• add focused loader/query coverage proving the helper resolves from compiler-shipped source;
• align docs/design.md, docs/architecture.md, and docs/progress.md with the resulting source/kernel boundary.

Explicitly excludes:

• math.abs, math.min, and math.max, which remain compiler-owned exact numeric overloads;
• math.average and math.median, whose generic numeric and empty-input contracts are not source-expressible without a separate decision;
• math.clamp, whose invalid-bound and NaN behavior remains host-dependent;
• math.gcd, math.lcm, and math.factorial, whose int64.MIN, overflow, negative-input, or saturation contracts need separate treatment;
• changing list ownership, generic numeric constraints, implicit coercion, module/import policy, or trusted stdlib origin;
• HIR, MIR, native code generation, or runtime ABI work.

Acceptance criteria

• Compiler-shipped stdlib source exports math.sum(values: list[int64]) returns int64 with the existing public call spelling.
• The dedicated math.sum hardcoded checker signature and Rust execution body are removed.
• Focused tests pin empty, singleton, positive/negative, and order-sensitive cancellation cases.
• Passing a list to math.sum retains the current consuming ownership behavior, including a compile-fail regression for reuse after the call.
• The int64.MAX + 1 case remains a deterministic runtime error after fix: report math.sum overflow #47; any source-boundary diagnostic wording is decided explicitly and pinned rather than changed accidentally.
• Existing argument/return-type diagnostics remain covered.
• A query or loader regression identifies compiler-shipped math source as the definition location.
• Retained primitive/overloaded math operations remain explicitly Rust-backed in design, architecture, and progress documentation.
• cargo fmt --check, cargo build, and cargo test -q pass.

Dependencies / open questions

Land or rebase on #47 so the intended overflow regression is present before removing the Rust dispatch arm. Coordinate the physical stdlib/math.jett module with #28 / PR #30 rather than creating a second competing math namespace file. The implementation should decide whether ordinary source arithmetic's integer overflow wording is sufficient or whether call-context diagnostics must retain the existing math.sum: prefix.

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