Solo stdlib: Map/dict builtins (map_new/set/get/has/keys/remove/len)

[hyperpolymath]

Sub-issue of #45. Highest-leverage gap. Add string-keyed Map builtins backed by Value::Record so tooling can use dynamic/computed keys (eval_field only supports static identifiers). Repro: /tmp/t_map.my in #45.

[hyperpolymath]

Implemented and merged to main (user signed off 2026-05-18). See #45 for the consolidated summary.