Add checked XRPC results with retry metadata #26

Adds a checked XRPC call path that preserves structured failure information instead of collapsing unsuccessful HTTP responses into transport errors.

Changes:

• add queryChecked / procedureChecked helpers returning XrpcClient.Result
• add XrpcError with HTTP status, ATProto error name/message, raw body, and rate-limit headers
• capture RateLimit-* and Retry-After headers in HttpTransport
• add conservative retry policy for 429, 500, 502, 503, and 504
• keep existing raw query / procedure response behavior intact

Verification:

• zig build -Doptimize=ReleaseFast
• zig build test -- --test-filter xrpc
• zig test src/internal/xrpc/transport.zig
• zig zen
• git diff --check