feat(ast): add Swift symbol parser

[ckorhonen]

Summary

Adds first-class Swift symbol extraction to CodeWiki’s AST parser registry. Swift files were already detected as source files, but there was no registered parser, so Swift/iOS repositories were effectively limited to file-level context. This adds a lightweight dependency-free parser that extracts common Swift declarations, imports, enum cases, properties, and coarse call/reference names.

Purpose

Make CodeWiki materially useful on Swift/iOS repositories without requiring SourceKit or adding a native tree-sitter Swift dependency.

Key Changes

• Add SwiftAstParser for best-effort Swift symbol indexing.
• Register/export the parser through the existing AST parser registry surface.
• Add regression coverage for Swift imports, protocols, enums/cases, structs, extensions, properties, methods, multiline function declarations, doc comments, and local-variable suppression.
• Include swift in the supported language registry assertion.

Test / Lint Results

• .venv/bin/python -m pytest tests/backend/test_ast_parser.py -q — 17 passed
• make test — 194 passed, 3 skipped, 1 warning
• make lint — backend Ruff and frontend ESLint passed
• make typecheck — mypy passed with no issues
• make build — frontend production build passed; Vite reported the existing large chunk warning
• Direct parser smoke over a Swift/iOS repository — parsed 102 Swift files and 4,214 Swift symbols

Linked Issues

None.

Screenshots / GIFs

N/A — backend parser/indexing change only; no UI changes.

Risk Assessment

Medium-low. This parser is intentionally heuristic and marked as swift_regex metadata rather than compiler-grade Swift analysis. It improves Swift repositories from file-only indexing to useful symbol-level context, but complex overload resolution, generated code, dynamic dispatch, and advanced Swift syntax should still be treated as best effort.