Add typed slice support and expand coverage

[danielgtaylor]

This follow-up PR finishes the remaining review-driven fixes and adds targeted regression coverage.

Summary:

• add direct support for common typed slices and reflection-backed arrays/slices across indexing, slicing, length, concat, in, contains, and where
• align typechecking with runtime for empty arrays and representative objects in where
• make lexer offsets and pretty errors rune-based so Unicode caret placement stays correct
• normalize nested numeric container equality and keep % runtime results consistent with folded expressions
• document numeric coercion and typed slice behavior in the README
• add focused tests for typed slices, reflection-backed arrays, truthiness, numeric conversions, helper formatting, and rune-aware errors

Validation:

• go test ./...
• package coverage raised to 80.0%

[Copilot]

Pull request overview

This PR extends the expression engine to handle common typed Go slices and reflection-backed arrays/slices throughout evaluation and typechecking, while also making error positioning Unicode-safe (rune-based) and adding regression tests/docs to lock in the behavior.

Changes:

• Add typed slice + reflection-backed array/slice support across indexing/slicing/length/concat and operators like in, contains, where.
• Switch lexer/token offsets and pretty error caret placement to rune-based positions for correct Unicode alignment.
• Improve numeric/container equality behavior and % runtime result consistency, plus add targeted tests and README documentation.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
typecheck.go	Treat empty arrays/objects in where as array[unknown] to match runtime behavior.
conversions.go	Add slice/array helpers (sliceLen, sliceItem, sliceRange, iterateSlice, etc.) and recursive deep equality normalization.
interpreter.go	Use new slice helpers for bounds checks, slicing/indexing, concat, in/contains, where, and make % return float64.
lexer.go	Track rune positions (runePos) and compute token offsets/lengths in runes.
error.go	Make Pretty() build rune-accurate caret lines and update docs to rune-based offsets/lengths.
README.md	Document rune-based offsets, numeric coercion for typed functions, and typed slice/reflection behavior.
lexer_test.go	Add tests for rune-based offsets/pretty errors plus token/node formatting and error accessors.
interpreter_test.go	Add tests for typed slices, reflection-backed arrays/slices, truthiness, numeric conversions, empty where inputs, and nested numeric equality.
conversions_test.go	Add focused unit tests for toNumber, generic slice append/concat helpers, and toBool.
review.md	Track review status/notes and residual gaps.

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

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