docs: track map helper extraction

[vycdev2]

Summary

• link the bounded map.is_empty, map.contains_key, map.set, map.get_or, and map.merge source-extraction work to enhancement: self-host compositional map helpers in stdlib #61
• record the current compiler-backed map boundary and retained equality, storage, cardinality, foundational lookup/update, heterogeneous-entry, and higher-order kernels
• keep the broader Phase D and Phase K stdlib migration status explicit

Verification

• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed (full workspace, including 428 driver fixtures)
• git diff --check upstream/main...HEAD — passed

Risk

• Low: documentation-only tracking links and boundary clarification; no compiler or language behavior changes.

Refs #61

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

[vycdev]

The documentation change looks good, but this branch now conflicts with main after #30 updated the same stdlib progress area. Please rebase and preserve both the completed math-helper extraction text and this map-helper tracking link.