Add error-tolerant AST construction for Atlas stack language
expressions and a URI parser for graph API endpoints.

Interpreter.syntaxTree builds a syntax tree from an expression
string, recovering from errors and collecting diagnostics for
each problem. The tree contains position info for all tokens,
resolved word references, and stack state at each point.

The URI parser extracts query parameters with precise positional
information so the AST and diagnostics can map back to exact
positions in the original URI. It handles percent-decoding while
maintaining raw-to-decoded offset maps for accurate spans.

New types in stacklang.ast:
- Token, ValueToken, CommentToken for tokenized input
- SyntaxNode (LiteralNode, WordNode, ListNode, CommentNode)
- SyntaxTree container with nodes, diagnostics, and final stack
- Diagnostic, Severity, Span for error reporting

New types in core.uri:
- UriParser for parsing graph API URIs with percent-decoding
- QueryParam with decoded text and raw-to-decoded offset maps