docs: track math sum extraction

[vycdev2]

Summary

• link the Phase D core stdlib status to the bounded math.sum extraction follow-up
• link the Phase K math status to the same tracking issue

Verification

• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed
• git diff --check — passed

Risk

• Low: documentation-only tracking links; no language or runtime behavior changes.

Tracks #63.

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[vycdev]

The documentation change looks good, but this branch now conflicts with main after #30 updated the same math/stdlib progress area. Please rebase and preserve the completed compositional math-helper extraction text together with this bounded math.sum follow-up link.