feat: improve error UX with suggestions, context, and source snippets

[alvinreal]

Polishes all error messages to be clear, actionable, and include context.

Changes

Fuzzy Suggestions

• Unknown format names suggest the closest match via Levenshtein distance
  • morph -f jsn → error: unknown input format: 'jsn'. Did you mean 'json'?
  • morph -t ymal → error: unknown output format: 'ymal'. Did you mean 'yml'?
• Unknown function names suggest the closest match
  • set .x = lowr(.x) → error: unknown function: lowr. Did you mean 'lower'?

Improved Cast Errors

• Cast errors now include: type name, value preview, path, and expected types
  • cannot cast array to int at .items: got [1, 2, 3], expected int-compatible type (int, float, string, bool, null)

Pretty Error Formatting

• pretty_print() method on MorphError for human-friendly stderr output
• Source snippets with line numbers and caret pointing at error column:

  error: unexpected token
    |
  2 | line 2 has error
    |     ^
  

• File errors include the full attempted path
• Parse errors show line:column location

Other Improvements

• Display impl for Path AST node (readable .users.[0].name format)
• main.rs uses pretty_print() for all error output

Implementation

• Levenshtein edit distance (single-row DP, no external deps)
• suggest_closest() generic fuzzy matcher (case-insensitive, max distance threshold)
• suggest_format() covers all format names/aliases
• suggest_function() covers all 50+ function names/aliases
• format_source_snippet() for source-annotated error display

Tests

• 16 new unit tests: edit distance, format/function suggestions, pretty printing, source snippets
• 6 new integration tests: CLI error UX for unknown formats, unknown functions, parse errors, file not found, cast errors

Fixes #31