Make AST spans and visitors reliable for source transformations

[michaelsutton]

Context

Argent uses the Sil parser and mutable AST visitor to transform embedded Sil source.

It uses the AST to identify semantic nodes. It then edits their ranges in the original source, reparses the result, and checks that the transformation is complete.

This process requires correct source spans and complete AST traversal.

Problem

Some composed expressions had incomplete spans.

For example:

(signed(next_status) + signed(increment)) as byte
(values)[index].length

Sil removed the grouping node before it built the outer expression. The resulting span could contain the closing parenthesis but omit the opening parenthesis.

Other API gaps required consumers to:

• wrap a statement in a synthetic function before parsing it;
• infer some type locations from containing nodes;
• maintain their own list of type-bearing AST forms;
• skip semantic elements when deserialized AST metadata had no source spans.

Change

This PR:

• preserves complete spans for composed expressions;
• exposes source spans for previously incomplete authored-type sites;
• adds parse_statement_ast for exactly one standalone statement;
• adds visit_type for AST-classified type occurrences;
• completes contract and struct traversal;
• visits semantic return types and attribute path segments even when source-span metadata is absent.

Plain parentheses remain normalized away. Exact inner spans remain available. For example, the identifier inside ((value)) still has the name span value.

Sil remains a semantic AST. This PR does not make it a concrete syntax tree and does not preserve comments or formatting through AST rendering.

Consumer

Argent PR #52 uses these APIs to remove source-transformation workarounds.

Argent will update its Sil revision to the merged master commit before it merges.

Validation

Tests cover:

• composed expression spans;
• authored-type spans;
• standalone statement parsing;
• all current type-bearing AST forms;
• nested contract and struct traversal;
• AST values deserialized without source-span metadata.