feat: support string-keyed maps

[danwt]

Summary

Specl only supports integer-range-keyed dictionaries (Dict[0..N, T]). String keys are not available, requiring manual mapping of named entities to integer indices.

Problem

Quint and TLA+ use string-keyed maps naturally:

// Quint
const resourceManagers: Set[str]  // {"rm1", "rm2", "rm3"}
var states: str -> ResourceManagerState
states.get("rm1") == Working

// Specl today
var rmState: Dict[0..2, 0..3]  // 0=rm1, 1=rm2, 2=rm3
rmState[0] == 0  // what is 0? which RM? which state?

Impact

Minor compared to sum types and sets — the integer-index encoding works fine mechanically. The main cost is readability: named entities like "rm1", "wolf", "goat" become anonymous integers.

Desired Behavior

Support string literal keys in dictionaries, or at minimum named constants that can serve as dictionary keys.

Context

Identified during Quint-to-Specl benchmark translation. All 7 translated specs used integer indices. See sidequests/quint-apalache-benchmark/README.md Section 3.1.

[danwt]

String-keyed maps would improve readability but the integer-index encoding works well mechanically. All existing examples use integer indices successfully.

Workaround: Use comments to document the integer-to-name mapping:

// 0=rm1, 1=rm2, 2=rm3
var rmState: Dict[0..2, 0..3]

This is lower priority than other improvements. Keeping open as a future enhancement.